What does it mean?
The idle air control valve regulates the engine’s idle speed. At idle, the throttle plate is of course closed. The IAC is computer-controlled and allows only a certain amount of air to bypass the throttle plate. This metered air controls the idle speed. The IAC has a “plunger” or “shutter” that moves to open or close the air bypass passage.
The more air the IAC allows to bypass the throttle plate, the higher the idle speed. The PCM (Powertrain Control Module) uses the IAC valve to maintain the correct idle speed under varying engine demands. It may be a three or four-wire sensor depending on the model.
The PCM monitors the voltages in the IAC circuits, and if it detects an incorrect voltage on one or more of the IAC motor circuits, it will set P0505. NOTE: Issues with the IAC would only be noticeable at idle or when the throttle is closed.
Symptoms
Symptoms may include:
MIL (Malfunction Indicator Lamp) illumination
Engine stalls and cannot idle unless the accelerator is pressed
Intermittent stalling when releasing the accelerator
Engine may idle too high
Causes
Potential causes of a P0505 code include:
Damaged IAC motor connector (check the connector and pins for damage and moisture)
One or more IAC circuits open or shorted to ground
One or more IAC circuits shorted to voltage
IAC has failed
PCM has failed
Possible solutions
This should be a fairly straightforward diagnosis. Start by clearing the error codes. Then disconnect the IAC and start the engine (this may cause other IAC codes to set; just ignore them for now). If the P0505 code does not reset, replace the IAC. It is internally shorting one of the IAC circuits. If the code returns after disconnecting the IAC and starting the engine, then you know it’s not the IAC motor. Visually inspect the wiring harness to the PCM. You may need to visually inspect the harness by removing all the wire insulation. It’s possible the wires are shorted together or with other wires in the harness.
If there are no wiring issues and the code keeps returning with the IAC disconnected, you can check the PCM by removing the IAC wires from the PCM connector (or cutting the wires at a point where they can be easily repaired), then start the engine (if you’re unsure, don’t do this. Take it to a qualified professional). Now, if the code does not reset, there is a problem with the wiring. Check continuity on all IAC wires. Any resistance indicates an issue. Check voltage on everything (at this point, there should be no voltage on the wires since they are cut at one end and disconnected at the IAC). Repair if necessary. But if the P0505 code resets after removing the IAC wires from the PCM connector (or cutting them), replace the PCM, as there is an internal issue with the IAC control circuit.
Other related DTCs: P0506, P0507