DTC Decoded: P1504 - Intake Air Control Circuit Malfunction

DTC P1504 Code Meaning: Intake Air Control Circuit Malfunction


  • Service Engine Soon Light illuminates
  • Rough idle
  • Pulsing idle
  • Hard cold start


  • Idle Air Control Valve harness is damaged
  • Idle Air Control Valve circuit has a bad electrical connection
  • Idle Air Control Valve is faulty

IAC valve

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When you see this code, there has been detection by the Powertrain Control Module (PCM) of an electrical load failure of the Idle Air Control (IAC) circuit. This means that there may be an open or a short in the harness, the connection may be damaged, or the IAC is dirty or faulty.

Your IAC valve is in control of the engine idle speeds and lets the air bypass the engine’s throttle plate. It is only in fuel-injected vehicles, and it is controlled by the information it receives from the ECM or PCM. If the valve is not operating properly, it can send too much air in. The PCM then will detect more air than it expects and trigger a check engine light. The PCM gets its data from a variety of sensors and then it sends out adjustment signals that affect the amount of air received by the engine. The IAC is not used at any speed over an idle, which is why you will notice a fault in it at startup or waiting in traffic. You may notice it before the check engine light is illuminated.

The IAC has airports that are sealed with gaskets that may begin leaking, which can cause the IAC to lose performance.

If the IAC is dirty from a build-up of carbon, it will not operate properly, which can result in a high idle speed, rough idle, or hard cold starting. A dirty IAC may also cause a pulsing idle. The bore air passages should be cleaned out any time you do a tune-up.