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Technical Description of OBD-II Trouble Code
Low Idle Air Control Circuit
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to all vehicles manufactured from 1996 onwards (Dodge, Chrysler, Hyundai, Jeep, Mazda, etc.). While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
When your OBD-II equipped vehicle displays a malfunction indicator lamp (MIL) and stores a code P0508, it indicates that the powertrain control module (PCM) has detected a problem with the intake idle air control (IAC) circuit. In this case, the value detected is lower than expected.
The IAC system revolves around a linear servomotor that controls a plunger. This plunger regulates the amount of air allowed to bypass the throttle plate and enter the intake manifold. By controlling the amount of air bypassing the throttle plate, the engine RPM at idle can be controlled. When the throttle plate is closed at idle, only a small amount of air is allowed to enter the engine through the throttle body. This small amount of air is insufficient for the engine to continue idling and would cause it to stall. To prevent this, an auxiliary air (vacuum) source is provided so that the engine can maintain idle even with the throttle plate closed. However, this auxiliary vacuum source would negatively impact engine performance and fuel efficiency when the throttle plate is open during acceleration. Therefore, a fully adjustable vacuum source controlled by the PCM is necessary.
The IAC valve plunger housing has two separate openings that allow air to pass through. The IAC valve is typically bolted to the intake manifold, near the throttle body. When the valve is open, ambient air flows through it and into the manifold. When the valve is closed, ambient air cannot flow through it.
An example of an Idle Air Control (IAC) valve:
The IAC valve is constantly in motion and rarely completely open or closed. It is controlled by the PCM, which uses input signals from the oxygen sensors, throttle position sensor, and other sensors to determine the appropriate amount of air to bypass the throttle plate. As the PCM adjusts the voltage and ground signals to the IAC valve, the valve responds accordingly. If the engine RPM levels do not align with the programmed desired limitations, a code P0508 will be stored and the MIL may illuminate.
Other trouble codes related to the idle air control system circuit include:
Severity & Symptoms
A malfunction in the idle air control system can cause the engine to stall or idle at an unexpectedly high RPM. This code should be considered severe and addressed promptly. Symptoms of a P0508 code may include:
Causes
Possible causes for this code include:
Diagnostic and Repair Procedures
A good starting point is always to check for technical service bulletins (TSB) specific to your vehicle. There may be a known issue with a known fix provided by the manufacturer, which can save time and money during diagnosis.
To diagnose a code P0508, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable vehicle information source (such as All Data DIY).
Begin by inspecting all IAC system wiring and electrical connectors. Repair any that are damaged, corroded, or burnt. Also, check the engine for any vacuum leaks that could contribute to uncontrollable engine RPM levels. If any vacuum leaks are found, repair them and recheck the system.
Next, connect the scanner to the vehicle diagnostic connector and retrieve all stored codes and freeze frame data. It is advisable to write this information down in case the code is intermittent. Clear the codes and test-drive the vehicle to see if the code is reset. If any other IAC related codes are stored, diagnose and repair those first. Additionally, check all technical service bulletins and safety recalls related to the vehicle and the specific malfunction. After performing any necessary repairs, drive the vehicle normally and allow the PCM to enter readiness mode before considering the problem resolved.
As a simple method for testing the IAC system, when the engine idle RPM is very high, carefully unbolt the IAC valve from the engine (while preserving the gasket or seal) and use a shop rag and fingers to control the engine RPM at idle. If it is possible to control a high idle condition after removing the IAC valve, it indicates that the IAC system is not functioning properly. If it is not possible to control the RPM at idle, there may be a large vacuum leak. Large engine vacuum leaks can include the PCV valve/hoses, vacuum hoses, vacuum brake booster, check valve or hoses, EGR valve stuck open, EGR gasket leak, or an intake manifold gasket leak.
Refer to your vehicle information source for the recommended method of testing the IAC motor with the DVOM. Replace the IAC motor/valve if it does not comply with the manufacturer’s specifications.
Additional diagnostic notes:
Professional Service
While we always recommend resolving this issue through mechanical means, if that is not possible, we offer the option to permanently delete this specific DTC. All other codes will continue to function. If you are interested in this option, please upload your ECU file to our portal.
