Very clear and impactful analysis. This is always the problem with convergence system, they are too intricate and cumbersome.
I have seen some YouTube videos and yes you are right a proper OBD2 professional scanner would point one directly to the faulty areas.
I have educated myself more using the link below, and you could see clearly that there are more suspect parts to check when the DTC's are related to the upstream A/F sensors than when all the DTC's are coming from both downstream sensors.. of course a digital multimeter would only give a rough accessment of what's happening, whilst the OBD2 scanner would show precisely a live information of the sensors behavior when the car is running. My question is how do I narrow down my problem in what you referred to as the 'SYSTEM', since the upstream sensors are the ones directly related to the overall systemic function of the engine, and they are NOT giving me any DTC's, except they have suddenly become insensitive and compromise their check duties. Will the following (vacuum leak, intake manifold gasket problem, IAC, EGR etc) cause DTC's only from the downstream sensors and bypass the upstream sensors?
Could you please point me to where I can download the repair manual for this car model, and would the repair manual list specifically the right part number should I need to buy new OEM sensors. The prefix for the downstream sensors I saw online is 89465.

Assuming the CAT is bad and sensors are fine, should I be worried about any mechanical or electrical faults that may have destroyed the CAT since the upstream sensors never reported any DTC's? (Just to be safe and not change the CAT with an imminent pending problem)
What is a FSM?

It's a Toyota avensis verso 2002 with 200,000 kilometers. 2.0L VVTi 1AZ-FE.

http://www.testroete.com/car/Toyota/mr2%20spyder/References/Technical%20Training/01%20-%20Engine%20and%20Engine%20Control/19.pdf

Let us take over the diagnosis and troubleshooting of your vehicle's problem.

Live online here on Nairaland.

Watch, Listen and learn.

The following are invited to the live online class:

Mojibola
Vicben
realGURU
Joshcoli
omega2128
phibetakappa
Shumuel
JesusCrew
uvalued
Kedra


If you are in this class and we don't know you, register your presence now LOL.

Olaboy1, we need some information from you to thoroughly fix the issue at hand.

Please provide the following:

Did you buy this vehicle brand new or is it new to you?
What is it's mileage?
Do you know its full service history?

How long have you had this problem?
Was any repair made before you had this problem?
Have you attempted any repairs so far? If yes, what exactly has been done to the vehicle?

What is the current status of the MIL
1. Do you have no driveability complaint along with the MIL? (Do you have any drivability issues?)
2. Do you have driveability complaint(s) along with the MIL? If yes, kindly explain. (Or is the MIL on but no drivability issues/complaints? )

Have you ever erased the DTC? Did it come back after that?


Freeze Frame Data required includes:
Fuel System Status (whether Open Loop or Closed Loop)
Calculated Load in Percentage
Short Term Fuel Trim Bank 1 in Percentage
Short Term Fuel Trim Bank 2 in Percentage
Long Term Fuel Trim Bank 1 in Percentage
Long Term Fuel Trim Bank 1 in Percentage
Engine Speed in RPM
Vehicle Speed in MPH or km/h
Engine Coolant Temperature in degrees
If a single-cylinder misfire is detected, indicate cylinder(s)

Current Parameter Display Information required includes:


Fuel System Status (whether Open Loop or Closed Loop)
Calculated Load in Percentage
Short Term Fuel Trim Bank 1 in Percentage
Short Term Fuel Trim Bank 2 in Percentage
Long Term Fuel Trim Bank 1 in Percentage
Long Term Fuel Trim Bank 1 in Percentage
Engine Speed in RPM
Vehicle Speed in MPH or km/h
Engine Coolant Temperature in degrees

Once we have your feedback, we will roll her into the service bay!

Picture Credits: https://en.wikipedia.org/wiki/Toyota_Ipsum

Picture of our first subject attached

Reporting to class. I bought the car used from a lady at around 211,000km, and the only thing I could make out of this is that the lady had no interest and knowledge about car care as there are many pointers to this. Very low radiator coolant, wrong wiper blades, no wiper water, low battery water level etc

I have gone into the service booklet and Toyota recommended service intervals are
1. Half service every 15,000km or every year change oil and oil filter
2. Full service and security controls like brakes etc every 30,000km or every 2nd year

The last recorded service history from the service book are full service at 150,000 and half service at 165,000. Can't say if she has been servicing it using regular mechanics without any record.

The car is just 2 weeks old with me and I want to put it in a perfect working condition.
Only repair I did was trying to replace the downstream oxygen sensors and cleared the codes and the check engine light came back.
Yes I have drivability issue along with the MIL. The RPM is too high at idle (1500rpm when cold at idle and around 700rpm when engine is warmed up), gas consumption as a result of sluggish movement when you try to overtake, you press the throttle pedal down to the max and the car is not responding in speed accordingly. More like something is stuck in the car's throat, little seconds delay when you try to crank the engine to start (I mean the car doesn't start at once, you need to hold the ignition key at the start position for like 5 seconds whilst the engine spins fine with good compression, my other car starts in 2 seconds and finally the engine vibrates little at idling position. The car was diagnosed by qualified govt emission control center and the checks came back with the following:
1. MIL
2. Idling CO/HC too high
3. Increased idle value incorrect (guess they are referring to the rpm at idle)
OBD2 error codes identified
P0161-O2 sensor oxygen circuit low voltage (bank 2, sensor 2)
P0141-O2 sensor heater circuit malfunction (bank 1, sensor 2)
P0340-Camshaft position sensor circuit malfunction
OBD self test not performed (I'm not sure what they mean by self test)

***observation****. As I was typing the above information about the old condition of the car, my mechanic plugged his laptop to scan for DTC's and the DTC's have totally changed from the previous information. I am attaching the new DTC's.
My mechanic doesn't want to spend his time in trying to diagnose the root cause of all the sensors problems, and I asked him if he has a scanner that can give us an oscilloscopic data of what's going on with the sensors, he said he doesn't have and I should just go ahead and buy new sensors. Mechanics are only interested in changing parts.
Can a OBD2 wifi scanner for Apple iPad solve my problem, or do I need to find another mechanic that has a more professional scanner to help me trouble shoot this problem. My mechanic did not even bother to check for vacuum leaks or any mechanical or electrical faults that could throw out those error codes. I have seen several videos on sensor problems, and directly changing the sensors is not a wise economic decision to take. Of course I understand with the current mileage of the car, the sensors should be changed anyway as a form of periodic service, but I want to be 100% sure I don't have other issues to address that could be something as simple as intake manifold gasket.
So guys now I have A/F upstream and downstream sensors issues
Bank 1 sensor 1 error code P1135
Bank 2 sensor 1 error code P1155
Bank 1 sensor 2 no error code
Bank 2 sensor 2 error code P0161

See attached

See 2nd attachment

Well done Olaboy.

You have good a good job of summing up the vehicle's history. Really helps in forging a path forward.

The vehicle is now in the service bay.

Let us study the issues and symptoms and prepare the next steps.

The class will resume between 10pm and midnight tonight.

Also invited to the class are the following:

critic007
castro
arcbaba
Andy790
abussyaya
consultancy
kjnwancaro
elbukaze

Until then, take care

Thanks AutoElectNG, here are the keys to the car, let me take a seat and watch the gurus diagnose the problems.
Strangely one mechanic is telling me to go buy new sensors and another one is saying the sensors are good just buy a new CAT, both mechanic haven't done any real systemic diagnosis.

Let's make it interactive.

Since we want to diagnose the vehicle from the perspective of OBD II, using the attached diagram, tell me which systems or components in the car are responsible for the following codes:


P0141

P0161

P0340

P1135

P1155

Let's start from here

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

P0141 will be auxiliary emission controls, computer output circuit
P0161 will be auxiliary emission controls, computer output circuit
P0340 will be ignition system or misfire
P1135 fuel and air metering, computer output circuit, ignition system or misfire
P1155 fuel and air metering, computer output circuit, ignition system or misfire

I don't get the P0340 again, the former mechanic for the lady said that has been fixed, and also no error code for P0141 (Just FYI)

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.

In the course of diagnosing/troubleshooting olaboy1's vehicle, this question will be answered.

But we would like you to revisit the question treated in the past:

What is a Diagnostic Trouble Code (DTC)? And why you should be interested in DTCs! here https://www.nairaland.com/3661728/on-board-diagnostics-obd-ii-questions#54360613

We want you to take special note of the definition of the expressions:

1. "Generic" or "Standard" OBD II codes

AND

2. "enhanced" or "OEM-specific" or "Vehicle Manufacturer" OBD II codes

Therein lies the key to determining which product to purchase.

Do I want to access 1 but not 2 OR do I want to access 1 and 2?


While #1 is defined by the SAE, #2 is defined by the Manufacturer.

Without access to the Manufacturer specific codes, you are losing out on a lot of data on the status of several extremely vital vehicle subsystems.


It will also mean that if those systems are having issues, you will have to go to a shop that has got such equipment to be able to get access to such data.

So unless you are a hobbyist who just wants to get by with minimal involvement, then you need to get a scanner that can read both.

You can either get the scan tool which your manufacturer promotes and has associated with its name with OR third-party tools with the capability to read manufacturer codes although not associated with the manufacturer.

Which way to decide depends on the vehicle you want to use the scanner on and how deep your pockets are and whether you want a scan tool that can only read and clear codes or you want full bi-directional communication (inclusive of the ability to reprogram the PCM/ECU).

--------------
MODIFIED BELOW
--------------

Think of it this way,

So you got a TV with just an antenna but it can only receive local channels
- that's generic data (what you get is what is available to everyone else)

OR

you just got a TV and you also paid for a satellite TV subscription, so you can watch local channels and foreign channels
- that's generic data and manufacturer specific data
- but they are several satellite TV subscriptions so the foreign channels you can have access to depends on the company you end up with and the subscription you paid for
- same thing manufacturer data is different from one manufacturer to the other: What data you can access depends on your vehicle, the scanner and the software that the scanner runs

olaboy1:
P0141 will be auxiliary emission controls, computer output circuit
P0161 will be auxiliary emission controls, computer output circuit
P0340 will be ignition system or misfire
P1135 fuel and air metering, computer output circuit, ignition system or misfire
P1155 fuel and air metering, computer output circuit, ignition system or misfire

I don't get the P0340 again, the former mechanic for the lady said that has been fixed, and also no error code for P0141 (Just FYI)

Well done.

But we want you to try again, using this link as a guide https://www.obd-codes.com/faq/obd2-codes-explained.php.

We prefer that you use this format.

Only then will we have conclusive proof that you are beginning to speak albeit understand fluent OBD II!
---------------------
MODIFIED BELOW
------------------
Since the above link is not working, two alternate links are provided below:

http://www.outilsobdfacile.com/data-trouble-code-obd2.php

http://www.myautorepairadvice.com/obd_codes.html

olaboy1:
Thanks AutoElectNG, here are the keys to the car, let me take a seat and watch the gurus diagnose the problems.
Strangely one mechanic is telling me to go buy new sensors and another one is saying the sensors are good just buy a new CAT, both mechanic haven't done any real systemic diagnosis.

Thanks for the keys, but we want you stick around and watch us do it!

We prefer not to comment on their diagnosis at this moment, all comments will be based on what we hear the PCM/ECU speak, since we can understand fluent OBD II.

The link is not working, can you check it out.. thanks

olaboy1:
The link is not working, can you check it out.. thanks

Post updated with two new links, recheck post in question above

Awesome now I've keyed in properly to what the codes are and exactly what parts of the vehicle are tied to each code family
P0140 powertrain code (P), generic fault(0), air fuel mixture(1), auxiliary emission(4), air fuel mixture(0)
P0161 powertrain code(P), generic fault(0), air fuel mixture(1), onboard computer and ancillary output(6), air fuel mixture(1)
P1135 powertrain code(p), Manufacturers fault(1), air fuel mixture (1), ignition system(3), engine idling(5)
P1165 powertrain code(p), Manufacturers fault(1),air fuel mixture(1), onboard computer and auxillary output(6), engine idling(5)
P0340 powertrain code(p), generic fault(0), ignition system(3), auxiliary emissions(4), air fuel mixture(0)

olaboy1:
Awesome now I've keyed in properly to what the codes are and exactly what parts of the vehicle are tied to each code family
P0140 powertrain code (P), generic fault(0), air fuel mixture(1), auxiliary emission(4), air fuel mixture(0)
P0161 powertrain code(P), generic fault(0), air fuel mixture(1), onboard computer and ancillary output(6), air fuel mixture(1)
P1135 powertrain code(p), Manufacturers fault(1), air fuel mixture (1), ignition system(3), engine idling(5)
P1165 powertrain code(p), Manufacturers fault(1),air fuel mixture(1), onboard computer and ancillary output(6), engine idling(5)
P0340 powertrain code(p), generic fault(0), ignition system(3), ancillary emissions(4), air fuel mixture(0)

Good Job.

Are you beginning to think of the vehicle as a system made up of subsystems and that the subsystems have components?

Now compare your answer above with mine below!

Is it a good repair procedure to clear DTCs in an OBD II vehicle? aka Please define Freeze Frame Data?


No!

The PCM/ECU does not just store DTCs.

It also stores the condition of the vehicle as at the moment in time that the PCM/ECU saw the need to trip on the MIL.

This is called Freeze Frame Data.

That is another way of saying that the PCM/ECU stores a “single point-in-time snapshot of the condition(s)” that caused the MIL to come one
Clearing the DTCs causes this valuable data to be gone for good impeding the troubleshooting process.

This data assists with troubleshooting for the reason that the technician can attempt to recreate the scenario as a means of determining what could have led the PCM/ECU to store such data.

I see your point clearly, the DTC's are good leads for technicians to use in troubleshooting.

P0141 MEANS


P = Powertrain Code (engine or transmission)
0 = Generic OBD Code defined by SAE
1 = Fuel & Air Metering
41 = Specific Code for the Exact problem of this ride, we need to consult the code books to tell us what which circuit and component is at fault
which yields the following information:HO2S B1 S2 HEATER CIRCUIT MALFUNCTION

P0161 MEANS


P = Powertrain Code (engine or transmission)
0 = Generic OBD Code defined by SAE
1 = Fuel & Air Metering
61 = Specific Code for the Exact problem of this ride, we need to consult the code books to tell us what which circuit and component is at fault
which yields the following information: HO2S S2 HEATER CIRCUIT MALFUNCTION

P0340 MEANS


P = Powertrain Code (engine or transmission)
0 = Generic OBD Code defined by SAE
3 = Ignition System or Misfire
40 = Specific Code for the Exact problem of this ride, we need to consult the code books to tell us what which circuit and component is at fault
which yields the following information:Camshaft Position (CMP) Sensor No Signal

P1135 MEANS

P = Powertrain Code (engine or transmission)
1 = Vehicle Manufacturer Special Code
1 = Fuel & Air Metering
35 = Specific Code for the Exact problem of this ride, we need to consult the code books to tell us what which circuit and component is at fault
which yields the following information: A/F Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)


P1155 MEANS

P = Powertrain Code (engine or transmission)
1 = Vehicle Manufacturer Special Code
1 = Fuel & Air Metering
55 = Specific Code for the Exact problem of this ride, we need to consult the code books to tell us what which circuit and component is at fault
which yields the following information: A/F Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)

Next issue is given our knowledge of vehicle systems, subsystems, components, actuators and circuits, which of these DTCs should we tackle first? And why?

This question is for you olaboy1

Following and learning.
I think we need to tackle the problems in the following order with reason
1. Circuit problem... the electrical condition of the oxygen sensors and camshaft sensor because they have to be in good working order to give us any reports
2. The combustion problem (air and fuel mixture)

olaboy1:
Following and learning.
I think we need to tackle the problems in the following order with reason
1. Circuit problem... the electrical condition of the oxygen sensors and camshaft sensor because they have to be in good working order to give us any reports
2. The combustion problem (air and fuel mixture)

Nice try.

We would start with P0340 because it deals with the ignition system and misfires.

Recall that you complained of delayed starting, erratic/unstable/high idle, sluggish overtaking, a non-responsive accelerator pedal and even poor MPG

These are the sort of problems that failures of components in the ignition system could cause.

Next, we would move to P1135 and P1155 because .......(can you fill in the blanks)?

Finally, we would conclude with P0141 and P0161 because .......(can you fill in the blanks)?

By the way, did you notice that you said that the report you shared indicated that an OBD Self-Test was not performed?

Ok cool P0340 first
The P1135 and P1155 would be next because they are responsible for air and fuel mixture, which would eventually send the right amount of generated heat in the CAT so as to prevent P0161 and P0141 passing there threshold.

P0161 and P0141 need to be fixed as they are responsible for final emission

Yes the report said self test was not performed, can you explain what that is

But the mechanic I use now will not be able to fix an ignition or misfire problem, because no way he can tell if that is fixed as the code is not showing up again. I don't know exactly what the old mechanic did to fix that P0340, how do we go about this as this is an integral part of engine function

Class dismissed for tonight.

Will resume tomorrow.

olaboy1:
Ok cool P0340 first
The P1135 and P1155 would be next because they are responsible for air and fuel mixture, which would eventually send the right amount of generated heat in the CAT so as to prevent P0161 and P0141 passing there threshold.

P0161 and P0141 need to be fixed as they are responsible for final emission

Yes the report said self test was not performed, can you explain what that is

Good job!

Will explain as we go along.

AutoElectNG:
Class dismissed for tonight.

Will resume tomorrow.

Alright have a good night rest. We continue tomorrow

Mojibola:
Brilliant piece.

Thanks for the compliments

Vicben:
This is interesting.

Welcome, hope you have not been missing classes!

realGURU:
Gud

Hope you have found the information valuable

Joshcoli:
Nice piece

Thanks for the compliments

kushfc:
Noted

Useful to you?

olaboy1:

What is a FSM?

Factory Service Manual