P0509 OBD II Trouble Code Idle Air Control Circuit High

How to diagnose, repair, and info on causes of a P0509 engine code which pertains to a high reading in the idle air control circuit. Also includes links to relevant forum topics on the code.

Symptoms of P0509 OBD II Trouble Code

  • High engine RPM level
  • Engine stall, especially at idle
  • Unusual noises from the throttle body area
  • Other IAC related codes

Common Causes of P0509 OBD II Trouble Code

  • Defective IAC motor/valve
  • Engine vacuum leak
  • Open or shorted IAC control wiring or connectors
  • Carbon buildup in throttle body or IAC valve
  • Faulty PCM or PCM programming error

Severity of P0509 OBD II Trouble Code

A malfunction in the idle air control system may cause the engine to stall or idle at an unexpectedly high RPM. This code should be treated as severe and addressed with urgency. Symptoms of a P0509 code may include: