Idle Air Control (IAC) System Diagnosis

Circuit Description
The Control Module (PCM/VCM) controls idle speed with an Idle Air Control (IAC) valve. The Desired RPM is based on sensor inputs and the actual engine RPM. Four circuits are used to control the IAC valve, allowing a precise amount of air into the intake manifold. In order to increase the idle RPM, the Control Module will command the valve pintle to move away from its seat allowing more air to bypass the throttle plate. To decrease RPM the Control Module commands the IAC pintle to move closer to its seat causing a lower airflow to bypass the throttle plate. A scan tool will display the Control Module commands to the IAC valve in counts. The higher the counts, the more air is allowed to bypass the throttle plate. Lower counts indicates less air is commanded by the Control Module.

IAC Valve Reset ProcedureWithout Scan Tool
1. Start the engine for 20 seconds.
2. Turn OFF the ignition for 10 seconds.
3. Start the engine.
4. Check for the proper idle speed.
With Scan Tool
1. Connect the scan tool to the DLC.
2. Turn ON the ignition leaving the engine OFF.
3. Using the scan tool, select the IAC Reset in Special Functions.
4. Activate the reset function and wait for the IAC counter to stop.
5. Start the engine.
6. Check for proper idle speed.

Diagnostic Aids
A slow, unstable, or fast idle may be caused by a non-IAC system problem that cannot be overcome by the IAC valve. The following checks should be made to repair a non-IAC system problem.

Important: If for some reason the air intake to the engine is restricted, the VCM will attempt to compensate by backing out the IAC pintle to maintain desired idle. If the engine has another source of air to the intake, the VCM will compensate by extending the IAC pintle, scan IAC counts may reach zero as the VCM tries to maintain desired idle.

^ Vacuum Leak (High Idle) - If idle is too high, stop the engine. Fully extend (low) IAC with tester.
Start engine. If idle speed is above 800 RPM, locate and correct vacuum leak including crankcase ventilation system. Also, check for binding of throttle blade or linkage.
^ Sticking throttle cable or throttle plate shaft.
^ System lean (high air/fuel ratio) - Idle speed may be too high or too low. Engine speed may vary up and down, disconnecting IAC does not help. May set DTC P0171. The scan tool will read Heated Oxygen Sensor (HO2S) output less than 300 mV (0.3 volt). Check for low regulated fuel pressure or water in fuel.
^ System rich (low air/fuel ratio) - Idle speed too low. Scan IAC counts are usually above 80. System obviously rich and may exhibit black smoke exhaust. May set DTC P0172. The scan tool will read Heated Oxygen Sensor (HO2S) signal fixed above 800 mV (0.8 volt).
Check for high fuel pressure, leaking or sticking injector. Silicone contaminated HO2S scan voltage will be slow to respond.
^ Throttle body - Remove IAC valve and inspect bore for foreign material.
^ IAC Valve Harness Connections - IAC valve connections should be carefully checked for proper contact.
^ Crankcase Ventilation Valve - An incorrect or damaged crankcase ventilation valve may result in an incorrect idle speed. Refer to Crankcase Ventilation System Diagnosis.
Refer to Rough, Unstable, or Incorrect Idle and Stalling. Rough, Unstable, or Incorrect Idle and Stalling
^ A/C Compressor - Refer to A/C Compressor Control Circuit Diagnosis if circuit is shorted to ground. If the relay is damaged, an idle problem may exist. Component Tests and General Diagnostics
^ If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance.

An intermittent may be caused by any of the following conditions:
^ A poor connection.
^ Rubbed through wire insulation.
^ A broken wire inside the insulation.

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Symptoms. Symptom Related Diagnostic Procedures

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
2. This test determines if the VCM is capable of closing the IAC valve.
3. This test determines if the VCM is capable of opening the IAC valve.
4. All engine mechanical related conditions should be checked and repaired before testing the IAC system.