- A bad IAC valve will affect the idle of the engine with a high or low idle speed.
- It will only affect the idle of the engine but engine will run well when the gas pedal is pressed.
- An IAC valve is controlled by the computer and allows air to bypass the throttle body butterfly plate.
What is an Idle Air Control Valve?
An Idle Air Control Valve (IAC) allows the computer to control the idle of the engine.
Vehicles with fuel injection have a throttle body that controls the air flow into the engine.
Inside the throttle body is a butterfly plate that opens and closes as the gas pedal is pressed and released.
When the gas pedal is released, the throttle body plate closes, and the vehicle’s computer takes over to maintain the correct air flow into the engine.
This is where the idle air control valve comes into play. The computer uses this valve to control the idle when the throttle body butterfly plate is shut.
NOTE: Some newer vehicles don’t use an IAC but have an Electronic Throttle Body (ETB) System, which controls the throttle butterfly plate.
Symptoms of a Bad Idle Air Control Valve (IAC)
- Idle Speed Erratic
- Idle Speed Too Low (Stalling)
- Idle Speed Too High
- Scanning the Computer Gives DTC Error Codes (P0505 to P0509)
If the engine does not idle correctly it may be caused by the IAC valve.
This is because when the gas pedal is pressed the idle air control valve is no longer in use, and the throttle body plate opens up, allowing more air in.
For example, if your vehicle idles too low, around 400-500 RPMs, and stalls out sometimes, but runs well as soon as you step on the gas, it’s a symptom of a bad idle air control valve.
This is often caused by a stuck open IAC valve that is letting in too much air.
- P0505: Code: Idle Control System
- P0506: Code: Idle Air Control System RPM Lower Than Expected
- P0507: Code: Idle Control System RPM Higher Than Expected
- P0508: Code: Idle Air Control Circuit Low
- P0509: Code: Idle Air Control System Circuit High
When the IAC fails, it will usually cause the codes (P0505 to P0509) to be given.
For example, if you use a scan tool and get a P0505 code (idle control system), there’s an issue with the idle air control valve.
NOTE: An intake air leak can cause the same symptoms as a bad IAC since it is letting air in around the throttle body.
How To Fix a Bad Idle Air Control Valve
If the IAC is cleaned, the engine may run badly for 2-3 minutes until the cleaner is burnt away.
The IAC may also have failed and need to be replaced.
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