OBD codes / P0508
P0508
ModerateIdle Air Control System Circuit Low
Drivable, but stalling is the real risk — an engine that dies at a stop or during a turn can catch you off guard. If it's stalling regularly, treat it as urgent; if idle is merely a bit rough, get it looked at soon and avoid leaving the car idling unattended.
What this code means
This code applies to older engines with a cable-operated throttle, where a small idle air control (IAC) valve meters air around the closed throttle plate to hold idle speed. P0508 means the electrical circuit driving that valve is reading lower than expected — typically a short to ground in the wiring, a shorted winding inside the IAC valve's motor or solenoid, or a corroded connector. With the circuit faulted, the computer can't manage idle airflow, so the engine may idle too low, stall when you lift off the throttle or drop into gear, or need a foot on the gas at stops. Carbon and oil vapor gunking up the IAC passages is the classic companion problem on these systems — it can overwork and eventually kill the valve — so cleaning is often part of the fix even when the valve is replaced. Newer drive-by-wire cars don't have a separate IAC valve; if you see this code on one, the diagnosis targets its idle control circuit per the manufacturer.
Symptoms you might notice
- Low or unstable idle
- Stalling at stops, when cold, or when shifting into gear
- Engine needs throttle to stay running at idle
- Idle that dips when the A/C or electrical loads kick on
- Check-engine light
What it costs to fix
Typical range: $30–$400 · about ~0.3–1 hrs of labor
| If the cause is… | Part | Labor |
|---|---|---|
| IAC valve cleaning | $8–$20 | ~0.3–1 hrs |
| IAC valve replacement | $50–$250 | ~0.3–1 hrs |
| Wiring / connector repair | $5–$80 | ~0.5–1 hrs |
The IAC valve is usually easy to reach on the throttle body, keeping labor short. Because P0508 is specifically an electrical-low fault, verify the wiring isn't shorted before replacing the valve — a new valve won't fix a pinched harness.
The price swings on which cause it turns out to be — so confirm the cause before paying. Diagnose P0508 for my exact vehicle →
Frequently asked
Can I clean the IAC valve instead of replacing it for P0508?
Cleaning fixes many idle problems on these systems and is always worth doing, but P0508 is an electrical fault — if the valve's winding is shorted internally, no amount of cleaning revives it. Clean it, test the resistance, and replace it if the electrical values are out of spec.
Does my car even have an IAC valve?
Only if it has a cable-operated throttle — roughly, most vehicles from the 1980s through the mid-2000s. Modern drive-by-wire engines control idle with the electronic throttle plate itself and have no separate IAC valve.
Why does my car stall with P0508?
At idle the throttle plate is closed, and the IAC valve is the engine's only air supply. When its circuit faults, that air path isn't being controlled — too little air and the engine starves and stalls, especially when the A/C, steering pump, or transmission adds load.
What's the difference between P0508 and P0509?
They're electrical opposites on the same circuit: P0508 means the IAC circuit reads abnormally low (usually a short to ground or shorted valve), while P0509 means it reads abnormally high (usually an open circuit, broken wire, or unplugged valve).
Seeing P0508 on your car? Get a diagnosis specific to your exact year, make and model.
Diagnose P0508 for my vehicleRelated trouble codes
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Generic OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.