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Consumer Assistance

How To Fix and Clear OBD-II Trouble Code P0505?

I failed the smog check. My car has a P0505 trouble code. The check engine light is not on. I have paid my registration fees and the car runs great. No clue what is going on.


OBD-II P0505 Trouble Code

Idle Control System Malfunction

Here is what P0505 means, in simple terms...

Your vehicle's engine is equipped with a very important component called the Idle Air Control valve. This component handles the task of controlling your vehicle's engine speed (RPM) while the engine is at idle, such as when anytime your vehicle is stopped but engine still running. The AIC will increase or decrease engine RPM as required. You may not even notice it working, but that means it's really doing it's job. Lets explain...

Why is it important to control engine RPM at idle?

Without an Idle Air Control valve an engine may very well stall (turn off). A properly functioning IAC valve should increase or decrease engine RPM, when so needed, to allow the engine to produce enough power to to efficiently run the various electrical/mechanical systems which the vehicle may be equipped with and which rely on engine power. Some examples are: the air conditioning unit, audio/video system, power steering system, secondary air injection system (EGR), misc accessories. An engine which can not provide the required power to run these systems will more than likely be performing under-efficiently and producing higher then normal amount of harmful emissions. It is basically an engine working hard to keep up with the demands of the rest of the vehicle. The engine will begin to stumble, hesitate to run smoothing, and probably cause some vibrations in the cabin.

At this precise point, and before you even notice the engine is about to start stumbling, the IAC kicks in and gives the engine a bit more air allowing it to increase its RPM and keep up with demand. The engine RPM will increase just enough to stop the engine from stalling, and will maintain proper RPM as required by the engine's manufacturer.

The more air the IAC allows into engine, the higher the engine RPM. The IAC is controlled Power Control Module (PCM). The PCM commands the IAC valve to keep idle speed correct with varying engine demands. The IAC contains a metal rod and plunger which moves to open or close position. The component is located at an air passageway within the engine throttle body.

What caused my vehicle to trigger a P0505 trouble code?

The PCM monitors voltages of the IAC circuits and if it detects an incorrect voltage on one or more of the IAC motor circuits it will set an OBD-II trouble code P0505. Problems with the IAC would only be noticeable at idle, when throttle is closed, and normally when the vehicle is stopped.

  • IAC motor connector is damaged. Check the connector and the connector pins for damage, and oil. The connector may be 3 or 4 pin.
  • Excessive carbon build-up in the IAC passages within the throttle body.
  • One or more IAC circuits open or shorted to either ground or 12V
  • Defective IAC
  • Defective PCM (not common)

What symptoms will my vehicle experience when trouble code P0505?

A P0505 trouble code will generally trigger the check engine light on. Depending on your vehicle's make, the check engine light may be referred to as the CEL, the Malfunction Indicator Lamp (MIL) or Service Engine Soon (SES) light. Freeze frame data containing information regarding engine sensor values at the time the ECM detected the P0505 fault will also be stored in the ECM's ROM memory and can be accessed via an OBD-II scan tool. Freeze frame data may be helpful to a technician when diagnosing the root cause of a P0505 trouble code when the fault is not easy to find.

  • MIL (Malfunction Indicator Lamp) or Check Engine light illumination
  • Engine stalls during stops and does not idle properly unless the gas pedal is depressed
  • Engine intermittently stalls when letting off the gas pedal
  • Engine may idle too high
  • Engine may idle too low

How can I fix a P0505 problem and where should I start?

Begin your P0505 diagnosis by clearing any fault codes stored within the PCM. Next inspect the IAC to ensure it is free from carbon and oil build-up. If you find the IAC clogged with debris, clean the component and reinstall. Start the engine with the IAC connected and inspect for trouble code P0505 to return. If this trouble code returns, continue on with this diagnosis. Unplug the IAC and start the engine. This will cause other trouble code to set but you may ignore them during your diagnosis. You will clear them later. If trouble code P0505 does not appear after starting the engine with the IAC unplugged, go ahead and replace the IAC. More than likely the IAC has an internal short. If trouble code P0505 does return after clearing the trouble codes, unplugging the IAC, and starting the engine, then the problem is not within the IAC motor, but rather the connectors and/or wiring harness between the IAC and PCM. We recommend inspect the connectors and harness. If you do not find any wiring problems, and trouble code P0505 keeps returning (with the IAC unplugged), we recommend checking the PCM. Check for continuity on all the IAC wires between the IAC and PCM. If trouble code P0505 code does not return after removing the IAC wires from the PCM connector, more than likely the problem is with the PCM. Defective PCM are not common and replacement should be a last resort.

Can I drive my vehicle with a P0505 trouble code and illuminated CEL?

Trouble code P0505 should be diagnosed and repaired as soon as possible. Not recommended, however, you may drive your vehicle while trouble code P0505 is present unless the engine stall or stumbling is is severe enough to hinder proper vehicle operation. Erratic or lack of idle control may be noticed. The most common problem is a vacuum line to one of the intake or throttle body ports. Not verifying that the engine does not have a vacuum leak before replacing the IAC. Not removing excessive carbon in the IAC passages if replacing the IAC valve.

posted by SmogTips Support

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