- Precautions In Diagnosing Faults

PRECAUTIONS IN IDENTIFYING DIAGNOSTIC TROUBLE CODES (DTCs)

NOTE: Do not disconnect connectors from the Powertrain Control Module (PCM), battery cables, PCM ground wire harness, or the 15 amp "TAIL" fuse. Such disconnection will erase PCM's memory.

Diagnostic information stored in the PCM's memory can be cleared as well as checked by using a scan tool. Before using a scan tool, read its operators manual carefully to have a good understanding as to what functions are available and how to use it.

PRIORITIES FOR DIAGNOSTIC TROUBLE CODES (DTC'S)

Troubleshooting priorities for multiple DTCs should be followed accordingly:

1. DTC's other than DTC P0171/P0172 (Fuel Trim lean/rich), P0300 to P0304 (Misfire), and P0400 (EGR system) should be followed first.
2. Next, DTC's P0171/P0172 (Fuel Trim lean/rich), and P0400 (EGR system) should be followed.
3. Finally, any Misfire DTC's.

Intermittent malfunctions
There are cases where the MIL indicates a DTC representing a fault which occurred only temporarily and has gone away. In such instances good parts are sometimes replaced unnecessarily. To prevent such an error, be sure to follow the instructions given below when using the "Diagnostic Tables."

When a malfunction can be identified, that is, it is not an intermittent fault
Check the sensor (actuator), wires and each connection to make sure that they are all in good condition.

When a malfunction cannot be identified, but the MIL indicates a DTC
Diagnose malfunctions by using the appropriate DTC charts. If the sensor (actuator), wires and connections are all in good condition ,erase the DTC from the PCM memory. Then conduct a test run and check what DTCs are present when the MIL illuminates. Only when it indicates a DTC again, is the malfunction present.

^ First failures of Type B DTC's will store in The Failed Since Code Cleared (Test Fail SC) but not in History. When the MIL is ON and a DTC is indictated in History, this indicates a current fault.

Notes on System Circuit Inspection
Most intermittent malfunctions are caused by faulty electrical connections or wiring. Perform careful checks of suspect circuits for:

^ Poor mating of connector halves, or terminals not fully seated into the connector body (backed out).
^ Improperly formed or damaged terminals. All connector terminals in malfunctioning circuits should be carefully reformed to increase contact tension.
^ Poor terminal to wire connection.
^ Never connect any tester (voltmeter, ohmmeter, etc.) to the Powertrain Control Module (PCM) when its connectors are disconnected. Attempting to do so may cause damage to the PCM.
^ Be sure to use a digital multimeter whose minimum resistance is more than one megohm per volt. Any other digital multimeter should not be used because accurate measurements are not obtained. (Use J 39200 or equivalent.)
^ When checking voltage at each terminal of the connector while it is connected to PCM, be sure to connect negative (-) probe to body ground and, using a jumper wire connect the positive (+) probe to connector terminal being tested. Applying probes of digital multimeter improperly may cause a sensor or PCM to be shorted and damaged.