Title: Advanced On-Board Diagnostics (OBD II) Strategies

Purpose: Familiarize participants with everything they need to know to diagnose post OBD II vehicles intelligently and professionally

Instructional Method:
1. Lectures
2. Discussions
3. Hands-on Practice
4. QnA sessions on Nairaland

Training Environment:
1. Classroom
2. Service Bay
3. ELearning (to reinforce concepts taught in 1 and 2 if you desire)

Materials Needed:
1. Pen
2. Pencil
3. Paper
4. Computer
5. Phone
6. Tablet

Training Aids:
1. PowerPoint Presentations
2. Videos
3. Computers
4. Handouts
5. Simulations
6. A variety of tech tools including but not limited to OBD Scanner and DMM (more advanced tools will be required later)
7. posts on Nairaland

What is On-board diagnostics (OBD)?


Wikipedia defines it as “an automotive term referring to a vehicle's self-diagnostic and reporting capability. OBD systems give the vehicle owner or repair technician access to the status of the various vehicle subsystems. The amount of diagnostic information available via OBD has varied widely since its introduction in the early 1980s versions of on-board vehicle computers. Early versions of OBD would simply illuminate a malfunction indicator light or "idiot light" if a problem was detected but would not provide any information as to the nature of the problem. Modern OBD implementations use a standardized digital communications port to provide real-time data in addition to a standardized series of diagnostic trouble codes, or DTCs, which allow one to rapidly identify and remedy malfunctions within the vehicle.”

Source: https://en.wikipedia.org/wiki/On-board_diagnostics

What is OBD II?

The second iteration of the OBD standard, i.e., An enhanced iteration of the OBD standard (capability-wise and standardization-wise)

Before OBD II, different manufacturer had different connectors, different pinouts, different electrical signaling protocols, and different messaging formats, non-standard DTCs.

In other words, you had purchase a different scan tool for every make of vehicle - imagine the cost implications for technicians and their shops in the days prior to OBD II

OBD II is an enhanced diagnostic monitoring tool that is integrated into vehicle PCM or ECU

OBD II allows your PCM or ECU to provide real-time data on its operations

Your PCM or ECU can generate a standardized series of diagnostic trouble codes (DTCs) which helps tremendously in the detection and remediation of faults in the vehicle

OBD II started life around 1994.

OBD II became mandatory in 1996 for all cars meant for the American market

It is basically an emissions standard and was initially designed for the reporting of vehicle emission status, but manufacturers latched onto the standard making it the de facto diagnosis and reprogramming protocol

If your vehicle's emission levels are greater than 1.5 times the originally certified emissions level, it with output a diagnostic trouble code (DTC).

The OBD standard was driven largely by the California Air Resources Board (CARB)'s mission of improving the quality of the environment - hence the statement that it was basically an emissions standard.

OBD II was built from the ground up sense failures in vehicle emissions control systems that would result in increased emission (read that to mean) pollution levels.

These failures could be electrical, chemical or mechanical.

With OBD II, if all systems (electrical, mechanical) except the catalytic converter are functioning perfectly, then there is a problem. A chemical failure. What chemical failure? A catalytic converter failure who is not doing its work.


https://www.youtube.com/watch?v=DXchXMVteyk

And what is the work of a catalytic converter? To chemically purify the air that passes through it! This failure is enough to cause the PCM/ECU to output a code.

This was not the case with OBD I, a chemical failure would not lead to a MIL!

The take-home lesson - OBD I brought drivability to the fore. OBD II brings emission (pollution) levels to the fore.

OBD II will however also output codes related to drivability - but that is not the primary reason why it is monitoring vehicle systems!



What this means is that irrespective of what you think or your friends and neighbors think, if your car was built to this standard, you are driving a modern car!

Brother and Sister, if your technician does not understand this language, you are in big trouble.

If your vehicle is a post 1996 vehicle. If your technician says don’t worry about the MIL, you are in even bigger trouble, because the car is telling you I got an issue, I need a checkup and you are ignoring it.

If your MIL was deleted, by your or the person who sold the car to you, then you know who your enemy is, how you intend to deal with him, well, I’ll leave that to you!


Got questions on OBD II? We will try to answer them in a down-to-earth manner so that anyone can understand!

Why do you need to have an OBD II Code Reader or Scan Tool?



To become aware of a developing problem before it becomes an issue aka early warning/alerting system for vehicle fault/issues

To retrieve and Display Diagnostic Trouble Codes (DTCs)

To erase displayed DTCs if the problem has been resolved or as part of the troubleshooting routine recommended by the vehicle manufacturer

To access vital information on the overall vehicle condition

To access vital information on the status of a particular vehicular sub-system or all vehicular sub-systems

To diagnose and troubleshoot vehicle faults/issues

To verify that repairs have actually fixed diagnosed faults

To access the various monitors in vehicle

To access and/or display freeze frame data

To access and display live data parameters

To record for later playback live data parameter in the course of a troubleshooting drive



Note: we have differentiated between a basic OBD II code reader
and a professional Scan Tool. In a subsequent post, the distinction between the two will be exhaustively treated. And which one you should buy!

Have you bought your OBD II code reader or professional Scan Tool?

Brilliant piece.

What is a Diagnostic Trouble Code (DTC)? And why you should be interested in DTCs!

A Diagnostic Trouble Code (DTC) is set in a vehicle's onboard computer when a fault occurs in any monitored system. The code number corresponds to the type of fault and can be used to diagnose the problem.

When an engine is running and the computer detects a problem in one of its sensor or output circuits, or even within itself, it will set a trouble code. In some systems, the trouble code number is retained in memory. In others, the trouble code is not stored but is regenerated when a mechanic runs the system through a special self-diagnostic test.

A trouble code will turn on the Check Engine Light (malfunction indicator lamp or MIL) to alert the driver the vehicle has a problem. To diagnose the fault, a code reader or scan tool is connected to the vehicle diagnostic connector to read the trouble code. Some tools display a definition for the code, while others show only a trouble code number. You then have to look up the definition of the trouble code number in a manual or a database to find out what it means. Listed below are links to "generic" OBD II codes and their definitions (codes that are common to all makes and models of vehicles). Vehicle manufacturers also use additional codes that are not in the generic code lists. These are called "enhanced" or OEM-specific codes.

On many older (pre-1995) vehicles, a trouble code can be read without a scan tool or code reader using a manual flash code procedure.
Once a trouble code has been found, the next step is to diagnose the fault.

NOTE: A trouble code by itself does NOT tell you which part to replace. You must diagnose the system, sensor and/or circuit to determine the fault before repairs are made or any parts are replaced.

Once you have a trouble code, the next step is figuring out why it set. Many codes are for systems, circuits or operating situations rather than individual sensors or other components, so additional tests are usually necessary to isolate and identify the fault that caused the trouble code to set. Scan Tool Companion is a computer reference program that can help you diagnose these kinds of faults. It includes takes the information on this page to the next level and provides guidance on which PIDs to look at on a scan tool when diagnosing a code, what to check when you have a specific code, and often provides "good" values to look for. Scan Tool Companion also provides diagnostic guidance by vehicle symptom and by system.

Courtesy: AA1Car read more here: www.aa1car.com/library/trouble_code.htm

Picture courtesy of: http://www.trouble-codes.com/

COMMENTARY:


In one sentence, the presence of a DTC is not conclusive evidence that your vehicle is faulty

It is absolutely necessary to troubleshoot the system, sub-system, sensor or circuit that the OBD II points to as being faulty because it might not be faulty (or just a part of the system not the entire system is faulty), and you may be dealing with a false alarm.


The difference between the pro (whether mechanic or DIY) and the amateur is the capability to diagnose the system intelligently instead of just throwing parts when the fault may be a wiring problem, a mechanical problem or a problem caused entirely by a different but related faulty sub-system that has no deteriorated enough to trip on its own DTC.

It has also severally been established that the OBD II systems in some makes and models are so sensitive that they set DTCs that should not be set. Once this condition has been well-established and investigations by the manufacturer have proved this to be the case, a software update has been made available to "desensitize" the on-board computer so it stops tripping on false DTCs.

------------------
MODIFIED
------------------

ISO 15031-6:2015
Road vehicles -- Communication between vehicle and external equipment for emissions-related diagnostics -- Part 6: Diagnostic trouble code definitions
https://www.iso.org/standard/66369.html

Diagnostic Trouble Code Definitions
http://standards.sae.org/j2012_201303/

OBD II Engine Diagnostics
http://www.aa1car.com/library/cm40122.htm

So how does one access OBD II information?

In one sentence, via the DLC interface which was created by the SAE J1962 specification.

For the details, please read on...

In an earlier post, we made it clear that OBD II brought sanity (read that to mean standardization of access) into the business of talking to and listening to the vehicle on-board computer.

The gateway (network-interface) to such communications is the OBD II Diagnostic Link Connector (DLC for short).

The DLC has 16 pins that provide access to a vehicle’s on-board diagnostics and live data streams.

The SAE J1962 specification started life in 1992 (as SAE J1962_199206) and has been revised 9 times ( the latest being SAE J1962_201607)

http://standards.sae.org/j1962_201207/

Just how does the interface look? See picture below.

You can read more here:

https://en.wikipedia.org/wiki/Data_link_connector_(automotive) and here
http://www.obdtester.com/obd2_connector and
https://en.wikipedia.org/wiki/On-board_diagnostics#OBD-II_diagnostic_connector

Picture credits: http://www.freeasestudyguides.com

So how does the onboard computer communicate OBD II information to the scanner?

In two short sentences, the PCM/ECU is multi-lingual and so speaks OBD II in several languages (protocols). The words of those languages are pulsed voltage signals aka serial data.

For the details, please read on...

The standard that governs this communication is SAE J1850 which started life in 1988 (as SAE J1850_198811) and has been revised 10 times (the latest being SAE J1850_201510).

http://standards.sae.org/j1850_198811/

The on-board computer communicates with your scanner via pulsed voltage signals (that is, serial data).

The communication can be via any of 5 protocols:

1. a single-wire Variable Pulse-Width (VPW) signal at a data rate of 10.4 Kbps on a 7-volt bus (SAE J1850 VPW) – GM loves to speak this one

2. a two-wire Pulse-Width Modulated (VPM) Signal at a data rate of 41.6 Kbps on a 5-volt bus (SAE J1850 PWM) – Ford loves to speak this one

3. asynchronous serial communication at a data rate of 10.4 Kbps on a 12-volt bus via a single wire (ISO 9141-2) – Asian, Chrysler and European vehicle tend to be fluent in this one

4. asynchronous serial communication at a data rate up to 10.4 baud on a 12-volt bus via a single wire (ISO 14230 KWP2000) – Asian, Chrysler and European vehicle tend to be fluent in this one as well

5. asynchronous serial communication at a data rate of either 250Kbit/sec or 500Kbit/sec on a CAN bus via two wires with the low signal wire running at 1.5-Volts and the high signal wire running at 3.5-Volts (ISO 15765-4 CAN) - American vehicles 2008 up speak this one by force of law while European vehicle 2003 up may speak this one. All things being equal, the world might be moving to a future in which the PCM/ECU will be fluent in just one language, this one!

Two key differences exist between each of these protocols: electrical and communication.

The first two signal types were proposed by the SAE while the last three were proposed by the ISO.

The signal type implemented in a vehicle depends on the design choices of your manufacturer.

In the 2015 revision of the SAEJ1850 specification, a High-Speed Mode (83.3 Kbps) for PWM and a 4x Speed Mode (41.6 Kbps) and Block Mode (unlimited data length) for VPW have been included.

http://standards.sae.org/j1850_201510/

COMMENTARY
If you buy a modern car, you are buying a more than a highly complex mechanical device, you are also buying highly computerized piece of equipment powered by hundreds if not thousands of electrical circuits and controlled by a number of ECUs

Does the distinction between a mechanic and an electrician (aka rewire) make sense in the circumstances?

Food for thought.

Further learning:
https://en.wikipedia.org/wiki/Controller_Area_Network

https://en.wikipedia.org/wiki/On-board_diagnostics#OBD-II_signal_protocols

http://www.obddiag.net/adapter.html

https://en.wikipedia.org/wiki/Automotive_electronics

Feedback welcome.

Questions welcome.

Suggestions welcome.

Ww

ok

This is interesting.

Ok Sir

Gud

Noted

Nice piece

Modern cars are seating on a lot of data. Are you taking advantage of that data?

We are into auto diagnostic equipment, scanners,key programmer, ecu programmer,car trackers etc. we also offer training services and after sales service.
Contact us 08034636279,08100981296. www.Masquest.net

My Toyota Avensis verso came with two power train codes P0161 and P0141, these are the two downstream oxygen sensors below the catalytic converter.
I have done some research online about O2 sensor codes and there are more than 20 reasons so far that could trigger my check engine to come on.
Using some simple logic, I need expert advice as I am meeting my mechanic today, if the upstream A/F sensors are bad then it means the car does not have the right equation and correct mixture of air and fuel, but what could be the major simple cause of both downstream O2 sensors throwing in those codes....
I feel if the car has vacuum leaks, bad fuel pump, ignition or timing problem or any mechanical fault that affects air and fuel ratio delivery to the combustion chamber, then the upstream O2 sensors should be the ones throwing in the error codes, who can explain precisely why I don't get error codes from the A/F sensors up the catalytic converter (CC), but rather from the downstream sensors.

olaboy1:

My Toyota Avensis verso came with two power train codes P0161 and P0141, these are the two downstream oxygen sensors below the catalytic converter.

I have done some research online about O2 sensor codes and there are more than 20 reasons so far that could trigger my check engine to come on.

Using some simple logic, I need expert advice as I am meeting my mechanic today, if the upstream A/F sensors are bad then it means the car does not have the right equation and correct mixture of air and fuel, but what could be the major simple cause of both downstream O2 sensors throwing in those codes....

I feel if the car has vacuum leaks, bad fuel pump, ignition or timing problem or any mechanical fault that affects air and fuel ratio delivery to the combustion chamber, then the upstream O2 sensors should be the ones throwing in the error codes, who can explain precisely why I don't get error codes from the A/F sensors up the catalytic converter (CC), but rather from the downstream sensors.

Is there a wiring problem between the downstream oxygen sensor and the PCM/ECU?

This wiring problem could be short ( wires that should not make contact with each other making contact) or an open (a cut along the path of what is supposed to be a continuous wire)

Is your oxygen sensor out-of-order and needs to be replaced?

Is your PCM/ECU bad and needs to be replaced?

Step 1. Confirm continuity of all the wires from the sensor to the PCM/ECU (check that there is no open)

Step 2. Confirm that there is no continuity where it should not exist between and wire and another wire from sensor to the PCM/ECU (check that there is no short)

Step 3. Check that the connectors are in order and that the wires are not bad at the connector end-point (e.g loose or cut or corroded or intermittent connection)

Step 4. If everything checks out with the wiring, then replace the sensor

Step 5. If after replacing the sensor, you still have problems then get a second and/or third opinion to confirm that the above issues are non-existent, if confirmed, then get a new PCM/ECU.

My bet: wiring problems that need to be fixed OR an A/F sensor that needs to be replaced (due to age or due to breakdown from other causes) not the PCM/ECU!

oh nice

Nice write up.

Brilliant, read until my brain got hot

Good job, broda!

Carry on! Following in 4D

olaboy1:
My Toyota Avensis verso came with two power train codes P0161 and P0141, these are the two downstream oxygen sensors below the catalytic converter.
I have done some research online about O2 sensor codes and there are more than 20 reasons so far that could trigger my check engine to come on.
Using some simple logic, I need expert advice as I am meeting my mechanic today, if the upstream A/F sensors are bad then it means the car does not have the right equation and correct mixture of air and fuel, but what could be the major simple cause of both downstream O2 sensors throwing in those codes....
I feel if the car has vacuum leaks, bad fuel pump, ignition or timing problem or any mechanical fault that affects air and fuel ratio delivery to the combustion chamber, then the upstream O2 sensors should be the ones throwing in the error codes, who can explain precisely why I don't get error codes from the A/F sensors up the catalytic converter (CC), but rather from the downstream sensors.

Seems u are confused about the operation of AF sensor,oxygen sensor 2 and air fuel mixture stoic.You can get error codes from any of the o2 sensors ,they work independently .So if you have DTC error codes on 02 sensor 2 ,try to fix it and the issue will go away .

AutoElectNG:


Is there a wiring problem between the downstream oxygen sensor and the PCM/ECU?

This wiring problem could be short ( wires that should not make contact with each other making contact) or an open (a cut along the path of what is supposed to be a continuous wire)

Is your oxygen sensor out-of-order and needs to be replaced?

Is your PCM/ECU bad and needs to be replaced?

Step 1. Confirm continuity of all the wires from the sensor to the PCM/ECU (check that there is no open)

Step 2. Confirm that there is no continuity where it should not exist between and wire and another wire from sensor to the PCM/ECU (check that there is no short)

Step 3. Check that the connectors are in order and that the wires are not bad at the connector end-point (e.g loose or cut or corroded or intermittent connection)

Step 4. If everything checks out with the wiring, then replace the sensor

Step 5. If after replacing the sensor, you still have problems then get a second and/or third opinion to confirm that the above issues are non-existent, if confirmed, then get a new PCM/ECU.

My bet: wiring problems that need to be fixed OR an A/F sensor that needs to be replaced (due to age or due to breakdown from other causes) not the PCM/ECU!

You are very knowledgeable and thanks for taking time out to respond.
I have done 50% of troubleshooting myself on the sensors so far and will finish the rest tomorrow.
1. I have checked the resistance in ohms on both downstream sensors and got 13ohms each, which are the correct readings for most Toyota cars. While the upstream sensors I got 1.2 ohms or so.
2. To be sure the PCM/ECU supplies current to the sensors, I have checked for 12v battery voltage from the wire harness and I got 11.9v been supplied to the upstream/downstream sensors, is it safe to rule out a PCM problem. FYI I changed both downstream sensors before with brand new ones and the CEL never went away, although brand new sensors don't always equal working or compatible sensors, as I later read after market parts can act funny if not designed to be 100% compatible with the car.

I forgot to check the ohms reading on those new sensors before returning them. The harness coming from the PCM has 4 wires, I only check one of the wires by connecting the positive lead on my multimeter to it and the negative lead to the battery terminal, what other things do I need to check on the wire harness since there are 3 more wires left. My sensor also has 4 wires (two heater wires, 1 signal wire and 1 ground wire), what are the 4 wires or 4 pins plug on the harness coming from the ECU.
3. I have NOT checked for continuity in the sensors.
4. I have not checked the voltage in the sensor when cold and when hot at around 2500rpm, guess that should be around 0.5v if I'm right. I need a repair manual to know the correct voltage I should get for this car model. Isn't a live oscilloscopic data from a scanner the best way to check that.

I am not getting any error code P0420 for a bad catalytic converter, of course I understand that a CAT may be destroyed by physical wear and tear and at such the upstream sensors won't throw out any codes if they are not experiencing any problems with fuel and air ratio mixture caused by any mechanical, electrical or vacuum leaks.

I saw little oil from the head gasket though and the car is not doing any overheating, my questions are 1. A blown head gasket will cause overheating right? 2. A leaking head gasket will not cause overheating right? But could cause oil leakage, and in that case what's the difference between a vacuum air leak and a head gasket oil leak and what different problems they can give to the car. Because a guy on YouTube said 90% of the time it's the intake manifold leaking or head gasket. Can a weak or leaking gasket cause downstream sensors to throw errors.

Now assuming all the sensor checks came back fine and good from PCM end to sensor end, what are the major causes of downstream sensors throwing out those codes. I know all the mechanical reasons for an upstream sensor giving error codes, but can they be the same reasons for the downstream sensors. Also note that the upstream sensors are mostly refered to as A/F sensors, while the downstream sensors are specifically called O2 sensors.

not many interested sha but i follow to be educater

honmusa:

Seems u are confused about the operation of AF sensor,oxygen sensor 2 and air fuel mixture stoic.You can get error codes from any of the o2 sensors ,they work independently .So if you have DTC error codes on 02 sensor 2 ,try to fix it and the issue will go away .

I know the difference between AF upstream sensors and O2 sensor 2 and how air fuel mixture stoic work. O2 sensors giving you error codes don't necessarily mean the sensors are bad, they are just there sometime to alert you of an imminent problem.
If you know why both downstream sensors are giving error codes and not the upstream sensors, I would appreciate your expert advice.
I am suspecting the catalytic converter because some of the symptoms of a bad CAT are what I am experiencing when driving on the high way, like difficulty in over taking a car, high rpm that don't correlate with the vehicle speedometer. And if a CAT is bad usually the downstream O2 sensors will pick on that since they are installed after the CAT, so that means the upstream sensors before the CAT will not give any error codes for a bad CAT.

AutoElectNG:


Note: we have differentiated between a basic OBD II code reader
and a professional Scan Tool. In a subsequent post, the distinction between the two will be exhaustively treated. And which one you should buy!

Have you bought your OBD II code reader or professional Scan Tool?

Pls can u show us here sample pictures of both OBDII code reader and d professional scan tool, probably recommended brands and their likely prices. Thnks.

JesusCrew:
Pls can u show us here sample pictures of both OBDII code reader and d professional scan tool, probably recommended brands and their likely prices. Thnks.

Your feedback is very much appreciated.

We are taking the training step by step.

We have have promised to do just that.

We are trying to do structured learning (step by step instead of haphazard training) which to a large extent does not exist in the Nigerian automotive training sector.

Before this week is over, we should have addressed this.
This is a major attempt to reverse that trend.

So watch this thread. We will address that question sooner than later

olaboy1:


You are very knowledgeable and thanks for taking time out to respond.

I have done 50% of troubleshooting myself on the sensors so far and will finish the rest tomorrow.

1. I have checked the resistance in ohms on both downstream sensors and got 13ohms each, which are the correct readings for most Toyota cars. While the upstream sensors I got 1.2 ohms or so.

2. To be sure the PCM/ECU supplies current to the sensors, I have checked for 12v battery voltage from the wire harness and I got 11.9v been supplied to the upstream/downstream sensors, is it safe to rule out a PCM problem. FYI I changed both downstream sensors before with brand new ones and the CEL never went away, although brand new sensors don't always equal working or compatible sensors, as I later read after market parts can act funny if not designed to be 100% compatible with the car.

I forgot to check the ohms reading on those new sensors before returning them. The harness coming from the PCM has 4 wires, I only check one of the wires by connecting the positive lead on my multimeter to it and the negative lead to the battery terminal, what other things do I need to check on the wire harness since there are 3 more wires left. My sensor also has 4 wires (two heater wires, 1 signal wire and 1 ground wire), what are the 4 wires or 4 pins plug on the harness coming from the ECU.

3. I have NOT checked for continuity in the sensors.

4. I have not checked the voltage in the sensor when cold and when hot at around 2500rpm, guess that should be around 0.5v if I'm right. I need a repair manual to know the correct voltage I should get for this car model. Isn't a live oscilloscopic data from a scanner the best way to check that.

I am not getting any error code P0420 for a bad catalytic converter, of course I understand that a CAT may be destroyed by physical wear and tear and at such the upstream sensors won't throw out any codes if they are not experiencing any problems with fuel and air ratio mixture caused by any mechanical, electrical or vacuum leaks.

I saw little oil from the head gasket though and the car is not doing any overheating, my questions are 1. A blown head gasket will cause overheating right? 2. A leaking head gasket will not cause overheating right? But could cause oil leakage, and in that case what's the difference between a vacuum air leak and a head gasket oil leak and what different problems they can give to the car. Because a guy on YouTube said 90% of the time it's the intake manifold leaking or head gasket. Can a weak or leaking gasket cause downstream sensors to throw errors.

Now assuming all the sensor checks came back fine and good from PCM end to sensor end, what are the major causes of downstream sensors throwing out those codes. I know all the mechanical reasons for an upstream sensor giving error codes, but can they be the same reasons for the downstream sensors. Also note that the upstream sensors are mostly refered to as A/F sensors, while the downstream sensors are specifically called O2 sensors.

Summary Comment:



Sorry for the non-response until now. Went out of town. Just returned.

Good job on your troubleshooting. We love it when drivers get down and dirty not waiting for technicians to do everything for them!

Point 1:
1. I have checked the resistance in ohms on both downstream sensors and got 13ohms each, which are the correct readings for most Toyota cars. While the upstream sensors I got 1.2 ohms or so.



Response 1:


Ideally, you should consult Toyota's Factory Service Manual for your Toyota Verso because it contains specific values that applies to your vehicle.

The values for one vehicle may not be correct for another vehicle even from the same manufacturer!

What model year is your Toyota Verso?

Electronics is related to mathematics. Everything is exact, there is no room for relative values.

Point 2:
To be sure the PCM/ECU supplies current to the sensors, I have checked for 12v battery voltage from the wire harness and I got 11.9v been supplied to the upstream/downstream sensors, is it safe to rule out a PCM problem. FYI I changed both downstream sensors before with brand new ones and the CEL never went away, although brand new sensors don't always equal working or compatible sensors, as I later read after market parts can act funny if not designed to be 100% compatible with the car.



I forgot to check the ohms reading on those new sensors before returning them. The harness coming from the PCM has 4 wires, I only check one of the wires by connecting the positive lead on my multimeter to it and the negative lead to the battery terminal, what other things do I need to check on the wire harness since there are 3 more wires left. My sensor also has 4 wires (two heater wires, 1 signal wire and 1 ground wire), what are the 4 wires or 4 pins plug on the harness coming from the ECU.



Response 2:


Are you saying that you conducted a continuity test for all the four wires of the sensors from the PCM/ECU to the A/F Sensors?

As well as a voltage drop test on all those four wires?

Could you let us in on how you conducted this test? Oh! You have told us how below. To do a good job, you need additional information, which we have lighted below.

When you bought a brand new A/F sensor? Did the part number match that which is in the vehicle? Or is it an acceptable substitute per manufacturer guidelines?

Did you compare values between the old and the new?

What is the mileage of this vehicle? Do you know its full service history?

What brand of A/F sensors did you buy? What of part numbers?

Each of the four wires from that A/F sensors ends up at the PCM/ECU. To test each wire properly, you will need a PCM/ECU pin-out diagram. It assigns all the ECU pins a number. It also tells you the function of each pin. You also need a wiring diagram because that way you can confirm or disprove that the vehicle has not been tampered with. A wiring diagram tells you the function of each wire, its colors and routing. And finally a factory service manual for detailed fix-it guidelines.


Point 3: I have NOT checked for continuity in the sensors.



Response 3:

You cannot fix wiring issues without checking that there is continuity where they ought to be continuity and that there is no continuity where there ought to be none! And to this right, you need access to a wiring diagram and a PCM/ECU pin out diagram.

Always remember to power down the vehicle ( battery disconnection) before doing the resistance test with your multimeter!

That is why we spoke about opens and shorts


Point 4:
4. I have not checked the voltage in the sensor when cold and when hot at around 2500rpm, guess that should be around 0.5v if I'm right. I need a repair manual to know the correct voltage I should get for this car model. Isn't a live oscilloscopic data from a scanner the best way to check that.



I am not getting any error code P0420 for a bad catalytic converter, of course I understand that a CAT may be destroyed by physical wear and tear and at such the upstream sensors won't throw out any codes if they are not experiencing any problems with fuel and air ratio mixture caused by any mechanical, electrical or vacuum leaks.



I saw little oil from the head gasket though and the car is not doing any overheating, my questions are 1. A blown head gasket will cause overheating right? 2. A leaking head gasket will not cause overheating right? But could cause oil leakage, and in that case what's the difference between a vacuum air leak and a head gasket oil leak and what different problems they can give to the car. Because a guy on YouTube said 90% of the time it's the intake manifold leaking or head gasket. Can a weak or leaking gasket cause downstream sensors to throw errors.



Now assuming all the sensor checks came back fine and good from PCM end to sensor end, what are the major causes of downstream sensors throwing out those codes. I know all the mechanical reasons for an upstream sensor giving error codes, but can they be the same reasons for the downstream sensors. Also note that the upstream sensors are mostly refered to as A/F sensors, while the downstream sensors are specifically called O2 sensors.


Response 4:

We like your spirit.

But we think you are running too fast and too far without a fit and proper guide to show you the way.

Your guide is the FSM. If you want to fix your ride, then you badly need a FSM.

A scope is a good piece of equipment to use, but it seems to us that we can fix this without having to use a scope.

We can do wonders with a multimeter and a scanner and the FSM.

We have decided to ignore this part of your response for now because:

1. you don't have a P0420 code and

2. in the initial post we asked that you completely rule out wiring issues, given the feedback received so far, that has not happened, we don't need to go on a wild goose chase, if you do the wiring system test and the vehicle or the suspect component fails, we can begin to consider other possibilities.

I am not getting any error code P0420 for a bad catalytic converter, of course I understand that a CAT may be destroyed by physical wear and tear and at such the upstream sensors won't throw out any codes if they are not experiencing any problems with fuel and air ratio mixture caused by any mechanical, electrical or vacuum leaks.

I saw little oil from the head gasket though and the car is not doing any overheating, my questions are 1. A blown head gasket will cause overheating right? 2. A leaking head gasket will not cause overheating right? But could cause oil leakage, and in that case what's the difference between a vacuum air leak and a head gasket oil leak and what different problems they can give to the car. Because a guy on YouTube said 90% of the time it's the intake manifold leaking or head gasket. Can a weak or leaking gasket cause downstream sensors to throw errors.

Now assuming all the sensor checks came back fine and good from PCM end to sensor end, what are the major causes of downstream sensors throwing out those codes. I know all the mechanical reasons for an upstream sensor giving error codes, but can they be the same reasons for the downstream sensors. Also note that the upstream sensors are mostly refered to as A/F sensors, while the downstream sensors are specifically called O2 sensors.

ADDITIONAL COMMENTS:

In an OBD II world, the more information you have from the PCM/ECU and other sources, the better your repair will be.

If there is a DTC, you need to find the suspect failed actuator, circuit, sensor, system or sub-system.

Next, you need to fix the problem following the manufacturer's guidelines for that problem.

Then clear the DTC.

And drive the vehicle to see if the DTC does not return (which means you have fixed the problem) and if the DTC returns, then you have not fixed the problem.

Repairs are more difficult if you don't have a component failure but a system failure. With a component failure, you simply replace the bad component, no need to troubleshoot! Interestingly, this is how to separate the men from the boys. The men understand how the system works, the role each component plays and can zero in on that knowledge to fix the problem.

In a best case scenario, to repair an OBD II vehicle you need access to

1. the owner's manual

2. the vehicle's factory service manual

3. Vehicle specific DTC reference guide obtained from your manufacturer instead of a generic SAE DTC reference specification - this is often
a deal breaker

4. Vehicle specific vacuum schematics

5. Vehicle specific vapor hose schematics

6. Vehicle specific wiring diagrams

7. Vehicle-specific guidelines for turning on various vehicle monitors (what minimum criteria needs to be met for the vehicle monitor to run)

8. Access to third-party repair databases (unlike in Nigeria, if you work in facilities that repair vehicles overseas, you will observe that shops subscribe to services such as these

Without all of this, or at least number 2, number 3 and number 6, you cannot do a good job of the particular problem at hand.

Since we have not yet treated what monitors are, you may read up on those here: https://www.obdautodoctor.com/scantool-garage/obd-readiness-monitors-explained

That not withstanding, we will revisit monitors sooner than later.

ECU PIN-OUT PICTURE REFERENCES:
http://www.mapecu.com/2014/05/mapecu-wiring-diagrams/ and http://www.digitalwebdirectory.com/sr20-ecu-pinout-wiring-diagram/