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P0505 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


If you connect your OBD-II scanner to a vehicle and notice that you have trouble code P0505, that is defined as “Idle Control System Malfunction.”


A P0505 trouble code is an OBD-II generic code that means that the engine control module (ECM) has determined the engine idle control system is not managing the idle to stay in a specific range. This simply means that the idle of the vehicle is too high or too low.


There are several key causes that can lead to the development of this issue and a P0505 trouble code. The first and one of the most common is that the idle air control (IAC) valve is carboned up. It may stick or have limited control due to the build-up of carbon.

This can also mean that the IAC passages are clogged or restricted by dirt, debris or carbon build-up. Finally, it could be possible that the engine has a vacuum leak in the intake system. This can either be after the throttle body, which the ECM cannot control with the IAC.


If you think that you may have a P0505 code, there are a few key symptoms that your vehicle might be having. Here are the things you should keep an eye out for that may indicate a P0505 trouble code may be present.

  • The Check Engine Light will be illuminated.
  • The code will be set in ECM memory.
  • The engine idle may be lower or higher than the vehicle specifications.
  • If it has a vacuum leak, the engine may idle roughly.
  • You may notice that the engine stalls when coming to a stop.
  • At times, the engine may idle too high.

If any or all of the symptoms arise, it is in your best interest to connect your OBD-II scanner to your vehicle to confirm the problem, if you have not already.


One of the biggest issues mechanics face when diagnosing and confirming a trouble code is that they don’t follow the necessary steps to properly assess the issue. This can lead to a persisting problem, replacing components that are working, or further damage to the vehicle.

Here are some steps you can take to properly diagnose a P0505 code: 

  • Scan trouble codes and document the codes in the ECM. Make sure that you clear and verify the codes are there, after you clear.
  • Take the car for a road test. Monitor the engine idle to see if it stays within specifications in and out of gear.
  • Check the engine for leaks in the vacuum.
  • Check the engine for carbon build up on the throttle plates and IAC valve inlet.
  • Disconnect the IAC to see if the engine’s base idle is within the vehicle specifications.
  • Remove the IAC and check for blocked passages.

Following these steps in order should lead to the proper diagnosis of the problem and avoiding any mistakes.

Common mistakes

However, there are some common mistakes associated with a misdiagnosis of P0505 code. The first is failing to verify if the engine does or does not have a vacuum leak before you replace the IAC. The next most common mistake is failing to remove excessive carbon in the IAC passages if you should need to replace the IAC valve

How serious is this?

Code P0505 is an indication the IAC is not working correctly. This can cause the engine to idle too low or high, which may cause the engine may stall when coming to a stop. This will cause the Check Engine Light to come on. If the check engine light is on, the vehicle will fail emission testing. The code may populate because the engine has a vacuum leak. A vacuum leak can cause the engine to run rough and give the vehicle poor gas mileage.

What repairs can fix the code?

There are a few simple ways you can correct this trouble code. The first includes replacing the IAC valve and cleaning the throttle body and passages. You may also have to remove carbon from the throttle body and IAC passages. Then you need to clean the IAC. This may also require that you repair a vacuum leak in the engine intake manifold vacuum system or gaskets.

Related codes

None listed.

Can you drive with a bad idle air control valve?

Although technically you can drive with a bad IAC, you should not continue to do so. Stalling may be caused by a bad IAC, which compromises vehicle safety.


Use this as a guide to give you a better understanding of how you can identify, diagnose, and fix a P0505. If you follow the steps for diagnosing the problem and avoid the common mistakes.

P0505 Code – What Does It Mean & How To Fix It