P0508 Idle speed control: signal or voltage too low

By: Diagno editorial team · Updated: 2026-10-15

P0508 means: Idle speed control: signal or voltage too low. The computer holds the idle speed steady with the throttle or an idle valve. OK to drive, but have it checked soon. The most common repair costs about €45–90 in Estonia, depending on make and model.

What it means

The computer holds the idle speed steady with the throttle or an idle valve. The speed is wrong or can't be held steady. The signal voltage is too low. Usually a wire is broken, a connector is loose or a wire is shorted to ground.

What you may notice

• Idle hunts or is too high or low
• The engine may stall
• Engine light on

What could be wrong

1. 1Connector loose, damp or corroded · Wiring or connector repair €45–90
2. 2Wire broken or shorted to ground · Wiring or connector repair €45–90
3. 3Faulty idle valve · Valve replacement €90–180
4. 4Engine computer fault (rare) · Control unit check and software €150–300

Repair difficulty: Easy

What you can check yourself

First look at the connector and wiring: anything loose, melted or green with corrosion? Cleaning the throttle body often helps. After cleaning or disconnecting the battery, the throttle often has to be relearned. You can do the simpler checks yourself.

How we fix it

1. We read the fault data on the diagnostic tool and check whether the fault is constant
2. We measure the circuit's supply, ground and signal
3. We repair the wire or replace the connector
4. If the wiring is fine, we replace the idle valve

Standard SAE description: Idle Air Control System Circuit Low