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 P0511 code, it means the powertrain control module (PCM) has detected a problem with the idle air control (IAC) valve circuit. If there are other more specific IAC codes, these should be diagnosed and repaired first.
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 plate 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 so the engine can continue idling with the throttle plate closed. Of course, the auxiliary vacuum source would harm engine performance and fuel efficiency when the throttle plate is open (during acceleration). Hence the need for a fully adjustable, PCM-controlled vacuum source.
The IAC valve piston housing is manufactured with two separate openings that allow air to pass through. The IAC valve is typically bolted to the intake manifold, on or near the throttle body. It is positioned so that when open (piston retracted from the opening), ambient air can flow through it 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 is 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 the voltage and ground signals to the IAC valve are adjusted (by the PCM), the valve responds accordingly. If engine speed levels do not match the programmed desired limits, a P0511 code will be stored and an MIL may illuminate.
Related idle air control system circuit trouble codes include:
- P0508 Idle Air Control System Circuit Low
- P0509 Idle Air Control System Circuit High
- P0518 Idle Air Control Circuit Intermittent
- P0519 Idle Air Control System Performance
Severity and Symptoms
A malfunctioning idle air control system can cause the engine to stall or idle at an unexpectedly high RPM. This code should be treated as severe and addressed urgently. Symptoms of a P0511 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 the throttle body or IAC valve
- PCM programming error or faulty PCM
Diagnostic and Repair Procedures
A good starting point is always to check the technical service bulletins (TSBs) for your specific vehicle. Your issue might be a known problem with a published fix from the manufacturer, potentially saving time and money during diagnosis.
To diagnose a P0511 code, you would need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information (All Data DIY will suffice).
Start by inspecting all electrical wires and connectors of the IAC system and repairing any that are damaged, corroded, or burnt. Also, check the engine for any vacuum leaks that could contribute to an uncontrollable engine speed level. If you find vacuum leaks, repair them and recheck the system.
Next, connect the scanner to the vehicle’s diagnostic connector and retrieve all stored codes and freeze frame data. It’s good practice to write this down in case the code is intermittent. Now, clear the codes and test drive the vehicle to see if the code returns. If other IAC-related codes are stored, diagnose and repair them first. Also, check all technical service bulletins and safety recalls for the vehicle and the specific malfunction. After performing all repairs, drive the vehicle normally and allow the PCM to enter readiness mode before declaring the problem fixed.
As a simple method to test the IAC system, when the engine idle speed is very high, carefully unbolt (to preserve the gasket or seal) the IAC valve from the engine and use a shop rag and your fingers to control the engine idle speed. If you can control a high idle condition after removing the IAC valve, you know the IAC system is not functioning correctly. If you cannot control the idle speed, suspect a significant vacuum leak. Major engine vacuum leaks can include PCV valves/hoses, vacuum hoses, brake booster, check valve or hoses, a stuck open EGR valve, EGR gasket leak, or an intake manifold gasket leak.
Consult 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 meet manufacturer specifications.
Additional Diagnostic Notes:
Disconnect all associated controllers before testing circuit resistance with the DVOM