What does it mean?
P1508 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Idle Air Control Valve Circuit Malfunction .
Symptoms
- Check engine light on with idle-related freeze frame data
- Idle too high, too low or continuously hunting
- Engine stalls when stopping, when the clutch is pressed or when a load switches on
- Long cranking and a stumble immediately after a cold start
- Engine speed does not settle after the A/C is switched on
- Slight increase in fuel consumption in stop-start traffic
Likely causes
- Heavy carbon build-up in the valve and the idle bypass passage, so the valve cannot reach the commanded position
- Open, shorted or high-resistance idle air control valve winding
- Corroded connector or a broken wire between the module and the valve
- Vacuum leak at the intake gasket, brake booster hose or crankcase ventilation line, forcing idle correction outside the learned range
- Dirty throttle bore, leaving the plate position outside the expected relationship with the valve
- Blown fuse or a bad relay contact on the valve supply
- Failed module driver as the least likely cause
How to fix it
- Read the idle position and idle correction values in live data and see how far they are from normal
- Remove the valve, inspect and clean the pintle, seat and bypass passage, then clean the throttle bore
- Carry out a smoke or spray test for vacuum leaks around the intake, booster hose and crankcase ventilation
- Measure the valve winding resistance and check the supply and drive circuits with the ignition on
- Inspect the connector and harness for corrosion, chafing and loose terminals
- Replace the idle air control valve if it is electrically faulty or mechanically seized
- Perform the idle relearn procedure and verify the correction value returns near the middle of its range