An idle air control (IAC) valve controls engine idle speed on vehicles with a throttle cable by letting a measured amount of air bypass the closed throttle plate. The engine computer opens it more when the engine is cold, when the air conditioning comes on or when the transmission is put in gear. Vehicles with electronic throttle control do not have a separate IAC valve; the throttle motor controls idle directly.
Bad idle air control valve symptoms
- Idle that is too high, too low or hunting up and down; see rough idle
- Stalling when coming to a stop, or when the AC compressor engages
- Hard starting, or starting only with the gas pedal pressed
- High idle after a cold start that never comes down
- Check engine light with idle codes
Idle control codes
| Code | Meaning |
|---|---|
| P0505 | Idle air control system malfunction |
| P0506 | Idle speed lower than expected |
| P0507 | Idle speed higher than expected |
What causes IAC problems
- Carbon buildup in the valve and its air passage, by far the most common cause
- A failed motor in the valve (many are small stepper motors)
- Vacuum leaks that add air the computer cannot control, making idle high (vacuum leak)
- Wiring or connector problems
How to clean an idle air control valve
- With the engine off, remove the IAC valve from the throttle body (usually two screws or bolts).
- Spray throttle body cleaner into the bypass passage and onto the valve pintle or rotor, and wipe away carbon; do not soak the electrical part of the valve.
- Clean the throttle plate and bore at the same time; see throttle body.
- Reinstall with a new gasket or O-ring, then follow the idle relearn procedure for your vehicle (some relearn by cycling the key, others by idling for a set time).
How to test an IAC valve
Many scan tools show the commanded IAC position or counts. Watch it while turning on the AC or shifting into gear: the command should change and idle should stay steady. A bidirectional scan tool can command the valve directly; if idle does not respond, the valve or its wiring has failed. If the computer is commanding the valve almost fully closed and idle is still high, look for a vacuum leak instead.
When idle problems are not the IAC
A misfire, low compression or incorrect cam timing also causes rough or low idle; see engine misfire and compression test. If a worn engine is behind persistent idle problems, compare replacements by code on our engines page.
More guides: engine stalling.
Trouble codes: P0508 Idle Air Control Circuit Low · P0509 Idle Air Control Circuit High · P0510 Closed Throttle Position Switch · all OBD2 codes.
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