FAQ - Questions regarding IAC, oxygen sensors, and throttle body position sensors

Idle Air Control Valve (IAC)

1. What is an Idle Air Control and what is its function in the car?

The idle air control valve (IAC or IACV) also called idle speed control (ISC) or stepper motor is used on fuel injected engines to control idle speed. The stepper motor type of IAC features a pintle, which is used to either block or open throttle air passages. The powertrain control module (PCM) controls stepper motor operation.

2. Which are the symptoms of a broken IAC/Stepper Motor?

Not surprisingly, a broken IAC often results in engine idle problems. An engine that is idling too fast or too slow may point to a faulty IAC. An engine that stalls out may indicate IAC problems as well.

3. How can I check, if my IAC/Stepper Motor is broken?

Typically, if there is a problem with the IAC or its control circuit, the PCM will set a diagnostic trouble code (DTC) and turn on the check engine light (CEL). If the light is on, you can plug a diagnostic scan tool into the diagnostic port under the dash and read the stored trouble codes. If the codes indicate a problem with the IAC or its circuit, a vehicle specific repair manual should be consulted for further testing.

4. How do I change the IAC/Stepper Motor in my car?

IAC replacement varies from vehicle to vehicle, but it is fairly straightforward in most cases. Vehicle specific repair information should be consulted before replacing your IAC. Typical IAC replacement is as follows:

  • Disconnect the negative battery terminal
  • Disconnect the IAC electrical connector
  • Remove the IAC valve attaching screws
  • Remove the IAC assembly
  • Install in the reserve order of removal

Oxygen Sensor

1. What is an oxygen sensor and what is its function in the car?

The front (upstream) oxygen sensor (O2)is one of the key inputs to the powertrain control module (PCM). It monitors the oxygen content in the exhaust stream, and relays that information to the PCM. The PCM then uses that information to control various outputs such as fuel control. The rear (downstream) oxygen sensor is used solely to monitor catalytic converter efficiency.

2. Which are the symptoms of a bad oxygen sensor?

A faulty oxygen sensor can cause numerous problems, the most common being a sudden increase in fuel economy and subsequent increase in emissions. Usually the check engine light (CEL) is illuminated as well. Other symptoms include, but are not limited to: a change in power and response, an engine that runs rough and poor idle quality.

3. How can I check, if my oxygen sensor is broken?

Typically, if there is a problem with the oxygen sensor or its circuit, a diagnostic trouble code (DTC) will be stored and the check engine light (CEL) will be illuminated. If the light is on, you can plug a diagnostic scan tool into the diagnostic port under the dash and read the stored trouble codes. If the codes indicate a problem with the oxygen or its circuit, the sensor can be confirmed faulty by monitoring its responsiveness via a scan tool or oscilloscope. If working properly, the front oxygen sensor should produce a waveform at idle that transitions rapidly between 0.1 volts and 0.9 volts. Unlike the front oxygen sensor, the rear sensor should hold a steady reading of approximately 0.45 volts if both the sensor and catalytic converter are working properly. As always, these are general testing procedures and a vehicle specific repair manual should be consulted for testing.

4. How do I change the oxygen sensor in my car?

Oxygen sensor replacement varies from vehicle to vehicle, but it is fairly straightforward in most cases. Vehicle specific repair information should be consulted before replacing your oxygen sensor. Typical oxygen sensor replacement is as follows:
  • Disconnect the negative battery cable.
  • Disconnect the oxygen sensor electrical connector.
  • Remove the oxygen sensor using a wrench, or preferably, a dedicated oxygen sensor socket.
  • Install in the reserve order of removal.

Throttle Body Position Sensor

1. What is a throttle position sensor and what is its function in the car?

Late model engines with fuel injection use a throttle position sensor (TPS) to inform the powertrain control module (PCM) about the rate of throttle opening and throttle position. The PCM then uses this information to control various outputs such as fuel control.

2. Which are the symptoms for a broken throttle position sensor?

A faulty TPS can cause numerous problems, the most common being stumbling or hesitation upon acceleration. Usually the check engine light (CEL) is illuminated as well. Other symptoms include, but are not limited to: a change in power and response, increased emissions, an engine that runs rough and poor idle quality.

3. How can I check, if my throttle position sensor is broken?

Typically, if there is a problem with the TPS or its circuit, a diagnostic trouble code (DTC) will be stored and the check engine light (CEL) will be illuminated. If the light is on, you can plug a diagnostic scan tool into the diagnostic port under the dash and read the stored trouble codes. If the codes indicate a problem with the TPS or its circuit, a vehicle specific repair manual should be consulted for further testing.

4. How do I change the throttle position sensor?

TPS replacement varies from vehicle to vehicle, but it is fairly straightforward in most cases. Vehicle specific repair information should be consulted before replacing your TPS. Typical TPS replacement is as follows:
  • Disconnect the negative battery terminal.
  • Disconnect the TPS electrical connector.
  • Remove the TPS.
  • Install in the reserve order of removal.
  • Adjust the TPS if required.