What Does It Mean?
This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to all vehicles from 1996 onwards (Dodge, Chrysler, Hyundai, Jeep, Mazda, etc.). Although generic, specific repair steps may vary by make/model.
When your OBD-II equipped vehicle has a malfunction indicator light (MIL) and a stored code P0508, it means the powertrain control module (PCM) has detected a problem with the idle air control (IAC) circuit, in this case a value lower than expected.
The IAC system centers around a linear servo motor that controls a piston regulating the amount of air allowed to bypass the throttle plate and enter the intake manifold. Controlling the amount of air bypassing the throttle plate helps manage engine idle speed. When the throttle is closed (at idle), very little air is allowed into the engine through the throttle body. The engine cannot maintain idle with this small amount of air and would likely stall. An auxiliary air (vacuum) source must be provided for the engine to keep idling with the throttle closed. Of course, this auxiliary vacuum source would harm engine performance and fuel efficiency when the throttle is open (during acceleration). Hence the need for a fully adjustable vacuum source controlled by the PCM.
The IAC valve piston housing is manufactured with two separate openings allowing air to pass through. The IAC valve is typically bolted to the intake manifold, on or near the throttle body. It’s positioned so that when open (piston retracted from the opening), ambient air can flow through into the manifold, and when closed (piston blocking the opening), ambient air cannot pass through.
An example of an idle air control (IAC) valve:
The IAC is rarely fully open or closed but remains constantly moving; controlled by the PCM. The PCM uses input signals from oxygen sensors, throttle position sensors, and various other sensors to determine how much air should bypass the throttle plate. When voltage and ground signals to the IAC valve are adjusted (by the PCM), the valve responds accordingly. If engine RPM levels don’t match programmed desired limits, a P0508 code will be stored and an MIL may illuminate.
Related idle air control system circuit trouble codes include:
P0509 Idle Air Control System Circuit High
P0511 Idle Air Control Circuit
P0518 Idle Air Control Circuit Intermittent
P0519 Idle Air Control System Performance
Severity and Symptoms
A malfunctioning idle air control system can cause engine stalling or idling at unexpectedly high RPM. This code should be treated as severe and addressed urgently. Symptoms of a P0508 code may include:
High engine RPM level
Engine stalling, especially at idle
Unusual noises from the throttle body area
Other IAC-related codes
Causes
Possible causes of this code include:
Faulty IAC motor/valve
Engine vacuum leak
Open or shorted IAC control wires or connectors
Carbon buildup in throttle body or IAC valve
PCM programming error or faulty PCM
Diagnostic and Repair Procedures
A good starting point is always checking technical service bulletins (TSBs) for your specific vehicle. Your issue might be a known problem with a published manufacturer fix, potentially saving time and money during diagnosis.
To diagnose a P0508 code, I would need a diagnostic scanner, digital volt/ohmmeter (DVOM), and reliable vehicle information source (All Data DIY would suffice).
I might start by inspecting all electrical wires and connectors in the IAC system, repairing any damaged, corroded, or burnt ones. I’d also check the engine for vacuum leaks that could contribute to uncontrollable engine RPM levels. If vacuum leaks are found, repair them and recheck the system.
Next, I’d connect the scanner to the vehicle’s diagnostic connector and retrieve all stored codes and freeze frame data. I prefer writing this down in case the code is intermittent. Then I’d clear the codes and test drive the vehicle to see if it resets. Obviously, if other IAC-related codes are stored, those need diagnosis and repair first. Also check all technical service bulletins and safety recalls for the vehicle and specific malfunction. After completing repairs, drive the vehicle normally and let the PCM enter readiness mode before declaring the problem fixed.
As a simple method to test the IAC system when engine idle speed is very high, I carefully unbolt (preserving the gasket or seal) the IAC valve from the engine and use a shop rag and my fingers to control engine idle speed. If I can control a high idle condition after removing the IAC valve, I know the IAC system isn’t functioning properly. If I cannot control idle speed, I suspect a major vacuum leak exists. Major engine vacuum leaks can include PCV valves/hoses, vacuum hoses, brake booster, check valve or hoses, stuck open EGR valve, leaking EGR gasket, or leaking intake manifold gasket.
Consult your vehicle information source for the recommended method of testing the IAC motor with the DVOM. Replace the IAC motor/valve if it doesn’t meet manufacturer specifications.
Additional Diagnostic Notes:
Disconnect all associated controllers before testing circuit resistance with the DVOM