Sensor Reference Guide


Oxygen Sensors (O2)
Throttle Position Sensor (TPS)
Mass Air Flow Sensor (MAF)
Engine Coolant Temperature Sensor (ECT)
Intake Air Temperature Sensor (IAT)
Exhaust Gas Recirculation (EGR)



Oxygen Sensors (O2)

Purpose:
Measure the amount of oxygen left in the exhaust stream to provide feedback to the computer about whether the air/fuel mixture is rich or lean.

Symptoms:
Poor fuel economy
Rich fuel mixture
Check Engine Light
Failed emissions test
Tech Tip: Test for O2 sensors in front of catalytic converter (S1B1 and S1B2 on your scantool).

With the engine warm, graph the O2 sensor output. Rev the engine to approximately 2000 RPM. A good sensor will oscillate from below 0.2 volts to above 0.8 volts, looking approximately like a sine wave. A bad sensor will be flat-lined and show no response to snapping the throttle. Test for O2 sensors behind the catalytic converter: Graph the rear and the front O2 sensor output. Rev the engine to approximately 2000 RPM. The front sensor will be oscillating actively. The rear sensor should show little reaction and stay at a midrange value (approximately 0.5 volts). If the sensor is flat-lined at zero volts, it has failed. If the rear sensor follows the oscillations of the front sensor, the sensor is good but the catalytic converter has failed.

Symptoms:
Oscillating front
Midrange back
Tech Tip: The computer compares the front and rear O2 sensors. A bad front O2 sensor can cause a rear sensor DTC. Never replace any O2 sensor without testing it first.


Throttle Position Sensor (TPS)

Purpose:
Tells the computer what the throttle position is

Symptoms:
Stumble or hesitation on acceleration
Test: With the key on, engine off, graph the throttle position sensor output. Gradually press the accelerator to full throttle. The graph should ramp smoothly with no spikes or drop-outs.

Tech Tip: Most Throttle Position Sensors wear out just off the idle position.


Mass Air Flow Sensor (MAF)

Purpose:
Measures how much air is flowing into the engine

Symptoms:
Poor fuel economy. misfires, stumbles, hesitation
Tech Tip: Graph the MAF sensor voltage on your scantool. With engine at idle, a steady voltage should be visible on the graph. Tap lightly on the sensor housing. A good sensor will not react to the vibration. If the graph jumps, the sensor is bad and needs to be replaced. Next rev the engine and verify that the graph climbs smoothly in proportion to engine speed. A jump or slow-reacting graph indicates a failed or dirty sensor.

MAF related trouble codes are often caused by a leak in the air ducts leading to the sensor. If the MAF sensor is fouled it can often be cleaned with an aerosol electronics cleaner.


Engine Coolant Temperature Sensor (ECT)

Purpose:
Tells the computer if the engine is warmed up

Symptoms:
Poor fuel economy
Poor performance
Tech Tip: After sitting overnight, the Engine Coolant Temperature should equal the air temperature. As the engine warms, it will rise steadily to 200 to 230 degrees Fahrenheit.


Intake Air Temperature Sensor (IAT)

Purpose:
Tells the computer the temperature of the incoming air used to calculate the air density and fine-tune the air/fuel mixture

Symptoms:
Poor economy, stalling or rough idle when cold
Tech Tip: The Intake Air Temperature Sensor will read approximately equal to the outdoor temperature when the vehicle is moving. Most common cause of trouble codes related to the IAT are a bad connection, or a sensor left disconnected after servicing the air filter.


Exhaust Gas Recirculation (EGR)

Purpose:
While the EGR doesn’t typically have its own sensor, the system is important and a common cause of Check Engine Light problems. The EGR system mixes a controlled amount of exhaust gas with the intake air to properly control the combustion process

Symptoms:
Failed emissions test, detonation (pinging) under acceleration
Tech Tip: If the EGR valve is vacuum controlled, graph the MAF and front O2 sensors. With engine at idle, manually open the EGR valve (do not manually move an electrically driven EGR valve) and watch for reaction of the MAF and/or O2 sensors. If there is no reaction, the EGR passageways or the valve itself are likely to be clogged and need to be cleaned.


Source: http://www.autotap.com/problem8_sensor_ref_guide.asp

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