Idle Air Control Circuit Intermittent
This code indicates that the PCM has recognized an inconsistency in the engine idle RPM.
Code Set Parameters
The PCM expects the engine to idle at a set RPM level. If the engine is idling excessively high or low, the PCM will recognize and attempt to correct the situation. If it cannot maintain the desired idle RPM, a code will be stored and the service engine light will be illuminated.
These may include a high or low engine idle RPM, engine stall at low speeds when the accelerator pedal is not depressed, engine stalls intermittently when releasing the throttle, a stored trouble code, and an illuminated malfunction indicator lamp
A carbon coked throttle body bore is by far the most common cause of this code. Other causes include shorted or open electrical circuits, a faulty idle air control valve, a large vacuum leak, or a bad PCM. PCM failure is rare.
Technicians report that replacing idle air control components prior to cleaning the throttle body bore can lead to a common misdiagnosis.
- When the engine is at idle, the throttle plate is in the closed position
- In this position very little air is allowed to enter the engine intake system
- The idle air control valve is actuated using an electric motor that is controlled by the PCM
- The valve opens in order to allow enough air to enter the engine intake, so that the desired engine idle RPM can be maintained
- When the throttle plate is opened, the PCM closes the valve thus eliminating the possibility of excessive air entering the intake, resulting in a lean exhaust mixture
- The valve is positioned over a clean air source and the engine intake manifold (usually near the throttle body)
- As the valve is opened, clean air is drawn into the intake where it contributes to a balanced air/fuel mixture
- When this supply of clean air is no longer required (the throttle plate is opened), the valve is either closed or greatly restricted, allowing the engine to maintain a similar air/fuel mixture at any RPM level.
Several tools will be instrumental in successfully diagnosing this code
- A suitable OBD-II scanner (or code reader) and a digital volt/ohmmeter will be most helpful in performing a successful diagnosis
Begin with a visual inspection of all wiring and connectors
- Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
- Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- Continue by clearing the code and operating the vehicle to see if it returns
- This will help to determine whether or not the malfunction is intermittent
After the codes are cleared, test drive the vehicle to see if the code returns
- If the code fails to immediately return, you may have an intermittent condition
- Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
Some manufacturers use a dedicated scanner program that guides you through an IAC system test, step by step
- If this is the case with your vehicle, then simply follow these guidelines until you come to a successful conclusion
- Otherwise, begin your diagnosis by clearing the trouble codes using a scanner or code reader
- Now disconnect the electrical connector from the IAC motor (see the manufacturer’s component locater to determine the position of the IAC valve/motor)
- Start the engine and see if this code returns
- If the code fails to return, replace the IAC valve because it has an internal electrical short
- If the code does not return, continue your diagnosis by visually inspecting system electrical wiring and connectors for open or shorted circuits from damage or corrosion
- Repair faults as required, reset the codes, and retest IAC operation
- If no obvious problems are detected in system wiring and electrical connectors, command the IAC motor to open using the scanner, then check for a voltage and ground signal at the IAC motor connector
- If there is voltage and ground present at the IAC motor, when the PCM commands movement, replace the IAC motor
- Cleaning the IAC motor may also be an option but the majority of the time replacement is a better option
- Chemicals tend to harm the electrical components and without using chemicals, the carbon debris found in the valve is virtually impossible to remove
If no voltage is detected at the IAC valve connector after commanding the motor open or closed with the scanner, turn the key to the off position and disconnect the PCM connector
- Using a digital volt/ohmmeter, measure circuit resistance and repair or replace wiring or connectors as required.