Idle Air Control (IAC) System Diagnosis

CIRCUIT DESCRIPTION
The engine idle speed is controlled by the idle air control (IAC) valve. The IAC valve is on the throttle body. The IAC valve pintle moves in and out of an idle air passage bore to control air flow around the throttle plate. The IAC valve consists of a movable pintle, driven by a gear attached to an electric motor called a stepper motor. The stepper motor is capable of highly accurate rotation, or of movement, called steps. The stepper motor has 2 separate windings that are called coils. Each coil is supplied current by two circuits from the powertrain control module (PCM). When the PCM changes polarity of a coil, the stepper motor moves one step. The PCM uses a predetermined number of counts to determine the IAC pintle position. Observe IAC counts with a scan tool. The IAC counts will increment up or down as the PCM attempts to change the IAC valve pintle position. An IAC Reset will occur when the ignition key is turned OFF. First, the PCM will seat the IAC pintle in the idle air passage bore. Second, the PCM will retract the pintle a predetermined number of counts to allow for efficient engine start-up. If the engine idle speed is out of range for a calibrated period of time, an idle speed diagnostic trouble code (DTC) sets.

DIAGNOSTIC AIDS
Inspect for the following conditions:
- High resistance in an IAC valve control circuit
- The correct positive crankcase ventilation (PCV) valve, properly installed and proper operation of the PCV valve
- Proper operation and installation of all air intake components
- Proper installation and operation of the mass air flow (MAF) sensor, if equipped
- A tampered with or damaged throttle stop screw
- A tampered with or damaged throttle plate, throttle shaft, throttle linkage, or cruise control linkage, if equipped
- A skewed high throttle position (TP) sensor
- Excessive deposits in the IAC passage or on the IAC pintle
- Excessive deposits in the throttle bore or on the throttle plate
- Vacuum leaks
- A high or unstable idle condition could be caused by a non-IAC system problem that can not be overcome by the IAC valve. Refer to Symptoms - Computers and Control Systems. Symptom Related Diagnostic Procedures
- If the problem is determined to be intermittent, refer to Intermittent Conditions. Intermittent Conditions

TEST DESCRIPTION

Steps 1-2:

Steps 3-5:

Steps 6-9:

Steps 10-11:

Steps 12-15:

Steps 16-19:

The numbers below refer to the step numbers on the diagnostic table.
5. This test will determine the ability of the PCM and IAC valve control circuits to control the IAC valve.
7. This test will determine the ability of the PCM to provide the IAC control valve circuits with a ground. On a normally operating system, the test lamp should not flash while the IAC Counts are incrementing.