Technical Description of OBD-II Trouble Code
Malfunction in the IAC (Idle Air Control) System
This diagnostic trouble code (DTC) is a generic powertrain code that applies to vehicles equipped with OBD-II. Although it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
What does that mean?
The IAC (Idle Air Control) valve is responsible for controlling the idle speed of the engine. When the engine is at idle, the throttle plate is closed. The IAC, which is computer controlled, allows a specific amount of air to bypass the throttle plate. This controlled airflow determines the idle speed. The IAC has a “plunger” or “shutter” that moves to open or close the bypass air passageway.
The more air the IAC allows around the throttle plate, the higher the idle speed. The PCM (Powertrain Control Module) uses the IAC valve to maintain the correct idle speed based on the engine’s demands. The IAC can be a three or four wire sensor, depending on the model of the vehicle.
The PCM monitors the voltages of the IAC circuits. If it detects an incorrect voltage on one or more of the IAC motor circuits, it will set the P0505 code. It is important to note that problems with the IAC are typically only noticeable at idle or when the throttle is closed.
Symptoms
The symptoms of this issue may include:
Causes
Potential causes of the P0505 code include:
Possible Solutions
Diagnosing and resolving this issue should be relatively straightforward. Begin by clearing the fault codes. Then, unplug the IAC and start the engine (this may cause other IAC codes to set, but ignore them for now). If the P0505 code does not reset, replace the IAC. It is likely internally shorting one of the IAC circuits. If the code DOES return after unplugging the IAC and starting the engine, then the issue is not with the IAC motor. Visually inspect the wiring harness all the way back to the PCM. You may need to remove the insulation from the wires to visually inspect the harness. It is possible that the wires could be shorting to each other or to other wires in the harness.
If there are no wiring problems and the code continues to return with the IAC unplugged, you can check the PCM by removing the IAC wires from the PCM connector (or clipping the wires in a location where they can be easily repaired) and then starting the engine (if you are unsure, it is best to take it to a qualified professional). If the code does not reset, there is a problem with the wiring. Check for continuity on all the IAC wires. Any resistance indicates a problem. Check for voltage on any of the wires (at this point, there should not be any voltage on the wires because they are clipped at one end and unplugged at the IAC). Repair as necessary. However, if the P0505 code DOES reset after removing the IAC wires from the PCM connector (or clipping them), replace the PCM. There is an internal problem with the IAC control circuit.
Other related DTCs: P0506, P0507
