With Vehicle Control Module (VCM-A)

IAC RESET
For diagnosis of Idle Air Control (IAC) refer to C Charts. Under Computer and Controls/System Diagnosis/Procedures. Testing and Inspection

If the IAC valve is disconnected or connected while the engine is running, the idle RPM may be wrong. The IAC valve may be reset by turning the ignition switch "ON" for 10 seconds, then "OFF" for 5 seconds.

The IAC valve affects the idle characteristics of the engine as well as throttle follow-up to compensation for sudden throttle closing. If it is open fully too much air will be allowed in the manifold and idle speed will be high. If it is stuck closed, too little air will be allowed in the manifold, and idle speed will be too low. If it is stuck part way open, the idle may be rough, and will not respond to engine load changes.

SYMPTOMS
System too 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 Diagnostic Trouble Code (DTC) PO171.

Scan tool and/or voltmeter will indicate a heated oxygen sensor output less than 300 mV (0.3 volt). Check for low regulated fuel pressure or water in fuel.

System too rich (Low air/fuel ratio) - Idle speed too low. IAC counts usually above 80. System obviously rich and may exhibit black exhaust smoke. Scan tool and/or voltmeter will read a Heated Oxygen Sensor (HO2S) signal fixed above 800 mV (0.8 volt) may set DTC P0172. Refer to Diagnostic Trouble Code Charts. Under Computer and Controls/System Diagnosis/Procedures. Testing and Inspection