P0508Idle Air Control System Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- CARB requires the idle speed control to be monitored: petrol engines must flag a fault if the target idle speed is not reached within +200/−100 rpm, diesels within ±30 %. Older engines use a separate idle air control valve that bypasses the closed throttle; with electronic throttle the throttle itself controls idle and no separate valve exists. P0508 is an electrical fault in the drive circuit of a separate idle valve. Subaru (also in the Saab 9-2X) feeds the valve from the main relay and pulses it to ground; the module counts signal edges and sets P0508 at 8 or fewer in 64 ms instead of about 160 – the output hangs at one level – with the warning lamp at once and limp mode without idle control. Many makers, Ford among them, report all circuit faults under P0511 instead.
- What you may notice
- Idle fluctuates or is wrong
- Engine stalls at idle
- Check engine light on at once
- Likely causes
- LikelyIdle valve coil faulty
- PossibleControl wire shorted to ground or open
- PossibleSupply missing (main relay, fuse)
- PossibleConnector contact at valve or module
- RareDriver in the engine module
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €50 – €350
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.