- Important Preliminary Checks

BEFORE USING THIS SECTION
Before using this section you should have performed the "On-Board Diagnostic (OBD) System Check" and determined that:

1. The PCM and Malfunction Indicator Lamp (MIL) "Service Engine Soon" are operating correctly.
2. There are no diagnostic trouble codes stored, or there is a diagnostic trouble code but no MIL "Service Engine Soon."

SYMPTOM
Verify the customer complaint and locate the correct symptom in the table of contents. Check the items indicated under that symptom.

VISUAL/PHYSICAL CHECK
Several of the symptom procedures call for a careful visual/physical check. The importance of this step cannot be stressed too strongly - it can lead to correcting a problem without further checks and can save valuable time. This check should include:

^ PCM grounds and sensors for being clean, tight and in their proper location.
^ Vacuum hoses for splits, kinks, and proper connections, as shown on "Vehicle Emission Control Information" label. Check thoroughly for any type of leak or restriction.
^ Air leaks at throttle body mounting area and intake manifold sealing surfaces.
^ Ignition wires for cracking, hardness, proper routing and carbon tracking.
^ Wiring for proper connections, pinches and cuts.

DO NOT use the Diagnostic Trouble Code (DTC) charts in "Engine Components/Wiring Diagrams/Diagnostic Charts," for intermittent problems. The fault must be present to locate the problem. If a fault is intermittent, use of DTC charts may result in replacement of good parts.

^ Most intermittent problems are caused by faulty electrical connections or wiring. Perform careful visual/physical checks.

Check for:

- Poor mating of the connector halves or terminal not fully seated in the connector body (backed out).
- Improperly formed or damaged terminal. All connector terminals in the problem circuit should be carefully reformed or replaced to insure proper contact tension.
- Poor terminal to wire connection. This requires removing the terminal from the connector body to check.

^ If a visual/physical check does not find the cause of the problem, the vehicle can be driven with a voltmeter connected to a suspected circuit. A Tech 1 scan tool can also be used to help detect intermittent conditions. An abnormal voltage or Tech 1 scan tool reading, when the problem occurs, indicates the problem may be in that circuit.
^ Loss of diagnostic trouble code memory. To check, disconnect Throttle Position (TP) sensor and idle engine until MIL "Service Engine Soon" comes "ON." DTC 22 should be stored and kept in memory when ignition is turned "OFF." If not, the PCM connections or the PCM is faulty.
^ An intermittent MIL "Service Engine Soon" with no stored diagnostic trouble code may be caused by:

- Ignition coil shorted to ground and arcing at spark plug wires or plugs.
- MIL "Service Engine Soon" wire to PCM shorted to ground (CKT 419).
- PCM grounds, refer to PCM wiring diagrams.

^ Check for an electrical system interference caused by a defective relay, PCM driven solenoid, electrical motors or switch. They can cause a sharp electrical surge. Normally, the problem will occur when the faulty component is operated.
^ Check for improper installation of electrical options such as lights, 2-way radios, etc.
^ Ignition Control (IC) wires should be routed away from spark plug wires, ignition system components, and generator. Wires from PCM to electronic Ignition Control Module (ICM) should have a good connection.
^ Check for open diode across A/C compressor clutch, and for other open diodes.
^ If problem has not been found, refer to "PCM Connector Symptoms" charts under "Diagnosis by Symptoms". Testing and Inspection
^ Check for improperly formed or damaged terminal on CKT 423. Refer to DTC 41. Powertrain Controls - ECM/PCM