P0506 OBD-II: Idle Air Control System RPM Lower Than Expected

OBD II Fault Code

• OBD II P0506

Fault Code Definition

• Idle Control System RPM lower than expected

The purpose of the Idle Control System is to manage the engine idle speed to meet the ever-changing loads placed on the engine, such as the use of air conditioning, heater blower motor, wipers etc.

Code P0506 sets when the idle speed is persistently below a pre-determned RPM level for a specified amount of time.

P0506 Symptoms

• Check Engine Light will illuminate
• Engine idle speed is very low
• Car will not idle when the engine is cold
• Car may stall at stop signs
• Engine idles rough
• In some cases, the engine may idle erratically

Common Problems That Trigger the P0506 Code

• Defective/sticking Idle Air Control Motor
• Intake Manifold Vacuum leaks
• Carbon buildup in the Throttle Body air passages
• Defective Power Steering Pressure Switch

Common Misdiagnosis

• Idle Air Control Motor is replaced when code P0506 was triggered by carbon buildup in the Throttle Body air passages and/or the butterfly valve of the Idle Air Control Motor.

Want to learn more?

The idle speed is often controlled by an Idle Air Control Motor mounted on or near the Throttle Body of the Intake Manifold. In earlier vehicles, an Idle Air Stepper Valve or Motor is used. After the model year 2000, more vehicles began to use a 'drive by wire system', whereby the Idle Air System is incorporated into the assembly of the Electronic Throttle Body. With any of the above systems, to raise or lower the engine Idle speed, the Powertrain Control Module (PCM) allows more or less air into the engine.

P0506 Diagnostic Theory for Shops and Technicians

Always study and record the freeze frame data. This tells what the engine running conditions were when code P0506 was triggered. The next thing I do is to try and duplicate the code setting conditions with a test drive. Then I study the data stream and look at the desired idle PID vs the actual idle rpm. Do they match? I place various loads on the engine, A/C, heater blower motor on full blast etc, and study how well the PCM can adjust and maintain the desired idle speed.

If the engine is running rough and lowering the idle speed, I look for the root cause(s). Is it a vacuum leak and fuel trim is very high? Is the EGR valve open when is shouldn't be?; Is the engine misfiring due to high secondary resistance? Is the throttle body caked with carbon?

If you suspect that the Idle Air Valve is at fault, be sure to lab scope the duty cycle signals from the PCM as you change engine engine loads. This will verify the proper functionality of the PCM as well as the wiring harness.