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Your vehicle’s idle control system is an important component of your car. This system prevents your engine from stalling when idle, especially when power-consuming accessories such as air conditioning, get turned on.

What happens when this system malfunctions? There are several OBD-II codes that could trigger because of a malfunctioning idle control system. One of these codes is P0506.

What Does the P0506 Code Mean?

Diagnostic Trouble Code (DTC) P0506 stands for “Idle Air Control System RPM Lower than Expected.” It means that your powertrain control module (PCM) has detected an engine idle speed lower than the specified revolution per minute (RPM) for a specific amount of time.

The P0506 trouble code can crop up in vehicles with electronic throttle control (ETC) or vehicles with a separate idle air control (IAC) valve. To control idle speed, an IAC valve regulates the amount of air that bypasses the throttle blade.

On the other hand, vehicles with ETC do not have a regular throttle cable nor do they have an IAC. As a result, they rely on an electric motor to control the throttle and regulate idle speed.

automotive pcm detecting problem in system close up
P0506 indicates that your powertrain control module (PCM) has detected an engine idle speed lower than the specified revolution per minute (RPM) for a specific amount of time.

P0506 is a generic DTC, which means various makes and models support it. Although it is a generic code, the diagnostic and repair steps for each case may vary depending on the specific vehicle that you own. For example, repair steps for a P0506 code for a Chevy may be different from the repairs steps for a P0506 code in a Ford.

For an extensive knowledge of idle control systems and how they’ve evolved over the years, you can read our technical discussion here. Otherwise, to quickly learn the possible causes of P0506, continue reading below.

Note: The definition of code P0506 may be different depending on the vehicle manufacturer. Consult the appropriate repair manual or repair database for the exact code definition.

What are the Possible Causes of the P0506 Code?

Here are the possible triggers of code P0506:

  • Faulty IAC valve
  • A problem within the ETC system
  • Vacuum Leaks
  • Dirty or defective throttle body
  • Collapsed air duct or a restricted air filter element
  • Circuit issues, such as damaged wires or poor connections
  • An issue with the PCM

What are the Common Symptoms of the P0506 Code?

Your check engine light will most likely illuminate once it detects a fault in your idle control system. However, it can illuminate for many reasons and it may be difficult to pinpoint which car part is causing the problem.

As a rule of thumb, get your vehicle checked by a professional when the check engine light activates. They would know how to further diagnose the problem. Aside from an illuminated check engine light, here are some other symptoms related to the code:

  • Low engine idle speed
  • Stalling
  • Rough Engine Idle
  • Engine may idle erratically (in some cases)

How to Diagnose the P0506 Code

Diagnosing the P0506 code may seem challenging, given that there are several reasons that could trigger this code. If you’re unsure what is causing this code, bring your vehicle to a mechanic for proper inspection.

If you’re planning to fix this code yourself, here is a video you can watch to get an idea of what the troubleshooting process might involve:

How to Fix the P0506 Code

As mentioned, the diagnostic and repair procedures for this code may vary depending on your car’s year, make, and model.

If you’re unsure about how these procedures work, you may consult vehicle repair manuals or bring your vehicle to an auto repair shop. If you do not have the DIY skill, it is best to leave the work to trusted professionals. They would know how to properly diagnose the problem and rule out any other underlying issue that is triggering the code.

A Quick History of Idle Control Systems

The old carburetor equipped vehicles were designed with somewhat limited idle speed control in the form of electromagnetic throttle kickers, dashpots that slowed the return of the throttle to the idle position, and idle load compensators that extended with intake manifold vacuum decay as the engine was idling. Idle speed controls like this were added due to power steering and A/C compressor load.

Motorized Idle Speed Control Motors

GM’s CCC and Ford’s EEC feedback carburetor systems (not to mention their foreign counterparts) began to add motorized idle speed control motors in the early 1980s that typically had an internal idle tracking switch and a reversible DC motor spinning a threaded sleeve around a threaded shaft so as to extend and retract a plunger resting against the throttle lever.

These motors do the same job as the old vacuum operated idle load compensators but are a bit more sophisticated than their older mechanical predecessors and require some algorithms in the engine controller so that when the throttle is released, the engine controller knows it needs to control throttle angle at idle. In addition to idle speed control, these motors also perform the “dashpot” function, preventing the throttle from snapping shut when it’s released (which typically stalls the engine).

End of the Carburetor Era

Near the end of the carburetor era, some carbureted systems began controlling the idle speed by using an engine-controller-manipulated stepper motor, allowing air to pass by the throttle plate, which enabled the computer to control the idle speed without actually moving the throttle plate. This became the order of idle speed control as throttle body injection caught on, with a few throttle body units still utilizing the old idle speed control motor design.

But by the late 80s, bypassing the throttle plate became the normal way of handling Idle Speed Control, even as MPI systems replaced throttle body injection.

two stepping motors and mechanical parts
Near the end of the carburetor era, some carbureted systems began controlling the idle speed by using an engine-controller-manipulated stepper motor.

GM and Chrysler/Dodge used a stepper motor that walked current through a series of windings to slowly extend and retract a plunger to allow and restrict airflow around the throttle plate to facilitate no-touch starting (maximum airflow on engine start), along with the standard ISC functions and the ever-present dashpot function to prevent stalling with sudden throttle release. Ford uses a two-wire Idle speed control that increases amperage to pull a plunger against a spring.

This kind of idle speed control depends heavily on engine RPM feedback as well as throttle position sensor angle. As soon as the key is switched on, closed throttle voltage is recorded and used to indicate C/T for that entire cycle.

OBD2 Era

As OBD2 became a thing in the mid-90s and became mandatory in 1997 (some one-ton trucks still had OBD1 systems as late as 1996), ISC remained mostly unchanged, except that the PX5xx series codes (like the P0506) appeared in the ECM/PCM code listing to indicate the inability of the idle speed control system to reach its target.

Turn-of-the century vehicles began to use electronic throttle bodies that eliminated the need for idle speed control units, since the ETC system took over that function. ETC equipped systems also don’t need a cruise control module for obvious reasons.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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