Engine

P1508 — Idle Air Control Overspeed Error

Severity: low Safe to drive

What P1508 means

The control unit detected that engine idle speed is higher than the target and the idle control system cannot bring it down. Normally the idle air control (IAC) valve or the electronic throttle meters just enough air at idle. Extra unmetered air, a stuck valve or a throttle held open by carbon will push idle too high and set this code.

Symptoms

  • High idle speed
  • Surging or hunting idle
  • Idle slow to drop after releasing the pedal
  • Jerky stops or shifts
  • Check Engine light on

Possible causes

  1. Vacuum leak (hoses, intake manifold, gaskets)
  2. IAC valve stuck open
  3. Carbon buildup holding the throttle plate open
  4. Wiring or connector fault at the IAC valve
  5. Idle relearn not done after cleaning or battery disconnect
  6. Control unit fault (rare)

How to diagnose

  1. Read all codes and freeze-frame data and compare actual to target idle speed
  2. Inspect vacuum hoses, intake ducts and gaskets for cracks and loose fittings
  3. Smoke-test the intake or carefully spray cleaner around it to find leaks
  4. Remove the throttle body and check it and the idle passage for carbon and sticking
  5. Check that the IAC valve moves freely and measure its coil resistance against spec
  6. Inspect the harness and connector for opens, corrosion and shorts to ground
  7. Check the throttle cable and pedal linkage for binding
  8. After repair, clear codes and perform an idle relearn

Common fixes

  • Repair vacuum leaks: replace hoses and gaskets
  • Clean the throttle body and idle air passage
  • Replace the IAC valve
  • Repair the wiring or connector
  • Perform throttle and idle relearn

P1508 on different brands

Dodge · manuals
For Dodge, the listed definition is a low voltage reading on the ambient/battery temperature sensor, so check that circuit.
Hyundai · manuals
On Hyundai it means the idle speed control actuator signal is high on coil #2.
Jaguar
On Jaguar it is a circuit malfunction of the idle air control (IAC) valve.
Kia · manuals
On Kia it means the IAC valve closing coil signal is high.
Land Rover
On Land Rover it is a circuit malfunction of the idle air control (IAC) valve.
Lexus · manuals
On Lexus the code refers to the PTC auxiliary A/C heater (disabled signal high), not idle control.
Mazda · manuals
On Mazda it is a short or open circuit in IAC valve 1.
MINI
On MINI it is an open circuit in the idle speed control valve opening solenoid control.

Questions and answers

What does P1508 mean?

It is an idle control code. Generally idle speed is above target because of extra air or a faulty IAC valve, though some brands define it differently.

Can I fix P1508 myself?

Often cleaning the throttle body and finding a vacuum leak solves it. If not, test the IAC valve and wiring, then perform an idle relearn.

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.