Technical Description of OBD-II Trouble Code
Intermittent Circuit Issue with Idle Air Control
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable 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 P0518, it indicates that the powertrain control module (PCM) has detected an intermittent problem with the intake idle air control (IAC) circuit. If there are any other more specific IAC codes present, those must be diagnosed and repaired first.
The IAC system revolves around a linear servomotor that controls a plunger, which regulates the amount of air allowed to bypass the throttle plate and enter the intake manifold. Controlling the amount of air allowed to bypass the throttle plate helps in controlling the engine RPM at idle. When the throttle plate is closed (at idle), only a small amount of air is allowed to enter the engine through the throttle body. The engine cannot continue to idle with such a small amount of air and would likely stall. To ensure that the engine can continue to idle with the throttle plate closed, an auxiliary air (vacuum) source must be provided. However, the auxiliary vacuum source would negatively impact engine performance and fuel efficiency when the throttle plate is opened during acceleration. Hence, there is a need for a fully adjustable vacuum source that is controlled by the PCM.
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, either on or near the throttle body. It is positioned in such a way that when it is open (the plunger moves out of the opening), ambient air is allowed to flow through it and into the manifold. Conversely, when it is closed (the plunger fills the opening), ambient air cannot flow through it.
An example of an Idle Air Control (IAC) valve:
The IAC is seldom completely open or closed but remains constantly in motion, controlled by the PCM. The PCM uses input signals from the oxygen sensors, throttle position sensor, and various other sensors to determine the amount of air that should be allowed to bypass the throttle plate. As the voltage and ground signals to the IAC valve are adjusted by the PCM, the valve reacts accordingly. If the engine RPM levels do not align with the programmed desired limitations, a code P0518 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 urgently. Symptoms of a P0518 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) for your particular vehicle. Your issue may be a known problem with a known solution provided by the manufacturer, which can save you time and money during diagnosis.
In order to diagnose a code P0518, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable vehicle information source (such as All Data DIY).
You can begin by inspecting all IAC system wiring and electrical connectors, and repairing any that are damaged, corroded, or burnt. Additionally, check the engine for any vacuum leaks that might contribute to an uncontrollable engine RPM level. If you find any vacuum leaks, repair them and recheck the system.
Next, connect the scanner to the vehicle diagnostic connector and retrieve all the stored codes and freeze frame data. It is recommended 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 there are any other IAC related codes stored, those need to be diagnosed and repaired first. Also, check all technical service bulletins and safety recalls for the vehicle and the specific malfunction. After performing any necessary repairs, drive the vehicle normally and allow the PCM to enter readiness mode before declaring the problem resolved.
As a simple method for testing the IAC system, when the engine idle RPM is very high, you can carefully unbolt the IAC valve from the engine (while preserving the gasket or seal) and use a shop rag and your fingers to control the engine RPM at idle. If you can control a high idle condition after removing the IAC valve, then it indicates that the IAC system is not functioning properly. If you cannot control the RPM at idle, there may be a large vacuum leak present. Large engine vacuum leaks may 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:
