Error Code P0509 is defined as Idle Air Control Circuit High, meaning the vehicle’s computer has picked up an inconsistency regarding the engine’s RPM when it’s in idle, which is commonly caused by shortages or leaks.

This is a generic trouble code which means it applies to all vehicles made since 1996 up to present and equipped with OBD-II system. It’s particularly more common among Chrysler, Dodge, Hyundai, Mazda, Jeep, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.

This code means the PCM (powertrain control module) has detected a problem in the IAC (Intake Idle Air Control) circuit, which in this case is value higher than its specifications.

Every vehicle has a set specification for its engine’s RPM when in idle. The PCM is in charge of monitoring the idle to make sure it stays within its specified range. If it determines that the engine’s idle is either too high or too low, it will try to correct the RPM on its own. If it’s unable to correct the RPM to do so, then that means there’s a problem with the vehicle. If the RPM is too high, then it will set the Error Code P0509 and activate the Check Engine light.

Other related trouble codes include:

Common Symptoms

  • High engine RPM
  • Engine stalls, especially at idle
  • Unusual noises coming from the throttle body

Other related IAC codes may also be present

Possible Causes

There are a few factors that lead to this code; the most common being is a bore throttle body or a chocked carbon coke. Other common causes include:

  • Sizeable vacuum leak
  • Defective IAC motor or valve
  • Electrical circuits issues in the IAC (open or shorted wiring or connectors)
  • Carbon buildup in IAC valve or throttle body
  • Damaged PCM (rare)

How to Check

To diagnose this code, connect an OBD-II scanner to the vehicle’s computer system. Check whether there are other codes present, take note of the data, and then reset the codes. Test-drive the vehicle to see if the code returns. If the code returns, then start to look at the possible causes, such as the IAC.

Check the throttle body bore. Make sure you clean it. Then, reset and then see if the code comes back on your test drive.

Clean the electrical connections and circuits in the IAC to make sure it’s working properly. Repair or replace connections as needed, such as burnt, corroded, or damaged electrical connectors.

Check the engine for vacuum leaks that might be contributing to uncontrollable engine RPM. Repair vacuum leaks and recheck the system.

In many cases, you may need to replace the IAC.

Or replace the PCM, but it’s extremely rare.

How to Fix

The most common repair for this code is addressing the problem in the electrical components of the IAC. Other common solutions will be:

  • Cleaning the throttle body bore
  • Repair of vacuum leaks
  • Replacement of the IAC
  • Replacement of the defective PCM (rare)

Make sure you consult with the vehicle’s information source for the recommended method of testing the IAC motor with the DVOM. If your findings don’t comply with the manufacturer’s specifications, then the common repair is to replace the IAC motor or valve.