With Vehicle Control Module (VCM)
IAC RESETThe 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, "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
When there is a problem with idle air control system or refer to Idle Air Control (IAC) Check. Refer to C Charts. Under Computer and Controls/System Diagnosis/Procedures.
^ 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 DTC P0171.
Scan tool and/or voltmeter will indicate an oxygen sensor output less than 300 mV (0.3 volt). Check for low regulated fuel pressure or water in fuel. A lean exhaust with an Oxygen Sensor (O2S) output fixed above 800 mV (0.8 volt) may be the result of a contaminated sensor, usually silicone. This may also set a DTC P0172. Refer to Diagnostic Trouble Code Charts. Under Computer and Controls/System Diagnosis/Procedures. Testing and Inspection
^ System too rich (Low air/fuel ratio) - Idle speed too low. Scan counts usually above 80. System obviously rich and may exhibit black exhaust smoke. Scan tool and/or voltmeter will read an Oxygen Sensor (O2S) signal fixed above 800 mV (0.8 volt).