Error Code P0505 is described as Idle Air Control (IAC) System Malfunction. In simpler terms, this error code is set when the IACV-AAC is not working properly.


The purpose of the Idle Control System is to control the idle speed of the engine. This is usually controlled by the IAC motor which mounted on or behind the Throttle Body of the Intake Manifold. An Idle Air Stepper Valve or motor is used in some cases. The PCM works by opening or closing either of these devices to raise or lower the engine idle speed, allowing more or less air to get to the Throttle Plate in the Throttle Body. This is performed to meet the changing loads put on the engine, such as using the heat, air-conditioning and wipers.

At idle mode, the throttle plate is obviously closed. The IAC is the one assigned to control and allow certain amount of metered air to the throttle plate, which also controls the idle speed. The IAC comes with a “plunger” or a “shutter” that moves to either open or close the bypass air passageway. When the engine is cold, the car will idle faster to keep the engine warm and help the Emission Control System maintain its operating temperature. The IAC then manipulates the speed through controlling the amount of auxiliary air that gets into the engine from the bypass passage. The more air that passes, the faster the idle.

The more air the IAC gets around the throttle plate, the higher the idle will be. The PCM helps in maintain the right ideal speed with varying demands of the engine. There can be a couple or more wire sensors depend on the model. The PCM also detects irregularities in the IAC circuits, and as soon as it detects the wrong voltage on, one or more of the IAC circuit will set out the Error Code P0505.  Issues on the IAC are usually only noticeable when the throttle is closed or at idle.

Common Symptoms

Some of the common symptoms of this error code involve:

  • Check Engine light is on
  • Stalling engine, requires hard press of the throttle to idle
  • Intermittent stalling when throttle is let off
  • Low engine idle speed, and engine dying at stop signs
  • Rough or high engine idle

Possible Causes

Some of the potential causes of this error code are:

  • Damaged IAC motor connector
  • Open or shorted to ground IAC circuit(s)
  • IAC circuit shorted to voltage
  • Failed IAC, faulty motor
  • Failed PCM
  • Vacuum leaks in the intake manifold
  • Noticeable buildup of throttle body air passage
  • Carbon buildup in the air passage of Throttle Body

How to Check

Diagnosis for this error code is pretty straightforward:

  1. Start your diagnosis by clearing the fault codes. After clearing the codes, unplug the IAC and start the engine. Now this will cause the IAC to set, simple ignore it for now.
  2. If the code DOESN’T reset, the problem is in the IAC, which means you need to replace it, as it is experiencing an internal short circuit.
  3. If the code DOES return after you unplug the IAC and start the engine, then the problem is NOT in the IAC motor.
  4. Next, inspect the wiring harness towards the ECM/PCM. This may require you to remove all the wire insulations, as it is highly possible that the wires are shorting each other or to other wires in the harness.
  5. If the code keeps returning even without wiring problems and the IAC unplugged, check the ECM or PCM by unplugging the IAC wires from its ECM or PCM connector. This can be a clipping connected in a where it can be easily repaired, and then, start the engine (if you’re not sure on how to do it, then DON’T risk it. Take your vehicle to a professional car technician instead).
  6. If the code DOESN’T reset, the problem is in the wiring system. Thus, check for the IAC wiring’s continuity. Any resistance will point to a problem. Also, check for voltage. Most often than not, there are voltages on the wires as one end is unplugged from the IAC and clipped on the other. Then, repair as required.
  7. If the error code still DOESN’T reset after you remove the IAC wires from the PCM connector, then the problem is in the PCM connector, and you need to replace it, as there is an internal problem on the control circuit of the IAC.

How to Fix

Repairs for this error code are already mentioned above, but to summarize, there are two important things to consider.

  1. Replace the IAC if damaged or broken
  2. Replace the PCM connector (clipping) if broken