Fuel and air

P0508 — Idle Speed Higher Than Expected

Severity: low Safe to drive

What P0508 means

The engine control unit (ECU) holds idle speed by controlling airflow through an idle air control valve or an electronic throttle. If actual RPM stays above the target and the ECU cannot bring it down, it sets P0508. This usually points to extra air entering the engine unmetered or a throttle that is not closing fully.

Symptoms

  • High idle speed
  • Surging or hunting RPM at stops
  • Car keeps rolling when you lift off the brake
  • Hard brake pedal at idle from a vacuum leak
  • Check Engine light on
  • Higher fuel consumption

Possible causes

  1. Vacuum leak: cracked hoses or a leaking intake manifold gasket
  2. Dirty or sticking throttle plate
  3. Stuck PCV valve or damaged PCV hoses
  4. Idle relearn not done after a battery disconnect or throttle cleaning
  5. Wiring short to ground in the idle control circuit
  6. Faulty idle air control valve or throttle actuator
  7. Rarely, a faulty ECU

How to diagnose

  1. Scan for codes and freeze frame data and look for related codes like P0171 or P0507
  2. Inspect vacuum hoses, intake ducts and gaskets for cracks and leaks
  3. Check the PCV valve and hoses and replace if stuck or damaged
  4. Inspect the throttle body for carbon buildup and make sure the plate moves freely and closes
  5. Check the throttle cable, if fitted, for tension at rest
  6. Inspect the idle valve or throttle wiring and connector for shorts and corrosion
  7. Compare live data: throttle position, target versus actual RPM and fuel trims
  8. Perform an idle relearn and see if the code returns

Common fixes

  • Repair vacuum leaks by replacing hoses, gaskets or seals
  • Clean the throttle body and perform the idle relearn
  • Replace the PCV valve or damaged hoses
  • Repair the wiring or connector
  • Replace the idle air control valve or throttle body assembly
  • Reflash or replace the ECU only after all other causes are ruled out

Questions and answers

Can I drive with P0508?

Usually yes, for short trips. A high idle raises fuel use and can reduce engine braking, so it is best not to put off diagnosis.

Why is my idle high after disconnecting the battery?

The ECU loses its learned idle settings. Warm the engine up and run the idle relearn procedure for your vehicle. If the idle stays high, look for a vacuum leak or a dirty throttle body.

Will cleaning the throttle body fix it?

Often it does, if carbon is keeping the plate from closing fully. Always do an idle relearn afterward or the RPM may stay off.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.