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  • The Diagnostic trouble code (DTC) P0505 has a simple definition of “Idle Control System.” This code can trigger when the idle air control system goes bad.
  • The P0505 code can be caused by a faulty idle air control valve, vacuum leak, damaged wires or connectors, an idle speed that isn’t calibrated, or a bad powertrain control module (PCM).
  • The issues setting the P0505 code can cause a check engine light, engine stalling, or idle speeds that are erratic, too high, or too low.

When your car’s primary computer, which is often referred to as the powertrain control module (PCM), perceives a problem with the idle air control system, it will warn the driver and record the appropriate diagnostic trouble code. One code that can come up on the scanner is P0505.

What Does the P0505 Code Mean?

Diagnostic trouble code (DTC) P0505 stands for “Idle Control System.”  The code may be accompanied by erratic idle speeds that can lead to stalling and other issues.

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idle control valve of a car
On most older vehicles, the idle control system relies on an idle air control valve (IAC) on or near the throttle body.

On most older vehicles, the idle control system relies on an idle air control valve (IAC) on or near the throttle body. The IAC meters the amount of bypass air around the throttle plate to sustain the idle speed.

When the vehicle is idling, the throttle plate closes, limiting the air supply to the engine. However, the engine still needs air to keep running.

Following instructions from the PCM, the idle control system opens the bypass air passageway. Air can flow around the closed throttle plate and reach the engine through this alternate route.

By adjusting the size of the bypass air passageway’s opening, the idle control system can manage the amount of air fed to the idling engine. It can raise idle speeds by admitting more air and reduce speeds by decreasing the flow.

The PCM keeps track of the current load on the idling engine. It gets the idle control system to tweak the air supply so that the engine gets the air supply it needs.

throttle body of a car engine
The PCM keeps track of the current load on the idling engine.

The idle air control system undergoes wear and tear like other car parts. When it goes bad, it will affect the engine’s idle speed, which could lead to the PCM issuing a P0505 code.

See also  Symptoms of a Bad Idle Air Control Valve

For more useful information that can help you come up with a DIY fix for code P0505, read our technical account of the development of idle control systems over time.

What are the Possible Causes of the P0505 Code?

Issues that affect the idle control system and trigger a code P0505 include:

  • A faulty idle air control (IAC) valve
  • A vacuum leak
  • An issue with the IAC circuit, such as damaged wires or poor connections
  • Idle speed out of calibration (relearn required)
  • An issue with the PCM, such as software in need of an update

What are the Common Symptoms of the P0505 Code?

Common symptoms of the P0505 trouble code include:

How to Diagnose the P0505 Code

Proper diagnosis is the key to fixing the P0505 code. However, determining the root cause of the issue might be challenging considering how many possible causes there are that could trigger the code.

You can leave the diagnosis to your mechanic or you can try to do it yourself using the help of a repair manual or an online guide. You can also refer to the video below to find out more about diagnosing the P0505 code:

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How to Fix the P0505 Code

Once you’ve figured out what caused the P0505 code, you can move on to fixing it. You could let your mechanic do the job or you can do it yourself with the help of repair manuals or online auto repair guides.

Take note that a fix that works for one particular car may not work for another model, especially one made by a different carmaker. Make sure to research confirmed fixes for P0505 that are specific to the year, make, and model of your vehicle. Remember to consult your owner’s manual before attempting any repair.

Other Notes About P0505

The P0505 code bears some resemblance to several other codes associated with the idle control system.

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About The Authors
Written By Automotive and Tech Writers

The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication. Holiday Campaign
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