Error Code P1506 is defined as Idle Air Control Overspeed Error. This code means that the engine idle is out of factory specification

This error code is a manufacturer-specific trouble code, meaning it does not apply to all vehicles makes. Rather, it applies only to specific vehicle makes, such as Ford. Specification on the definition, troubleshooting, and repairs still vary from one make and model to another.

Definition

The IACV or Idle Air Control Valve assembly works by controlling the engine’s idle speed and provide a dashpot function. The IAC valve assembly meters intake air around the throttle plate through a bypass within the IAC valve assembly and throttle body. The PCM (powertrain control module, also known as ECM or engine control module) determines the desired idle speed or bypass air and signals the IAC valve assembly through a particular duty cycle. The IAC valve then responds by positioning itself to control the amount of bypassed air. The PCM monitors engine RPM and increases or decreases the IAC duty cycle for it to achieve the desired RPM.

Error Code P1506 is set when the PCM detects an engine idle speed that is out of the specifications (higher RPM) set by the manufacturer. Sometimes, the IAC can have a carbon build up.

Common Symptoms

As with other error codes, this error code activates the Check Engine light and registers the code to the system.

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Possible Causes

Possible causes for this code include:

  • Defective Idle Air Control (IAC) valve
  • Open or shorted IAC valve harness
  • Poor electrical connection in IAC valve circuit
  • Intake leaks
  • Stuck PCV valve

How to Check

The first thing you need to do is help the vehicle re-learn the idle. You can do this by disconnecting the battery, removing the valve, cleaning and re-installing the valve and then driving the vehicle for around 5 minutes with both headlights and A/C on for the vehicle to relearn the idle.

Also, check for leaks in the intake or check whether the PCV valve is stuck.

How to Fix

It is important not to start with an idle air control valve (IAC) replacement when you have this code, because in many cases the problem is caused by lots of vacuum leaks, and can cause serious idle problems. Air leaks into the engine after the MAF has already metered the incoming air, causing the O2 (oxygen) sensor to see very lean exhaust. Since the vehicle is at idle, the computer will try to reduce incoming air by shutting the IAC down. When it commands for less air, and the vehicle still idles high, it will set the error code.

In this case, the usual cause of the problem is air leaks in the rubber hose.

Start your diagnosis by replacing both the PCV and the hose. Many people report that their vehicle doesn’t stop this hose.

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If the problem persists, then you need to replace the IAC valve.