Diagnostic System Check - With 4L80 Transmission

On-Board Diagnostic System Check:

MIL (Service Engine Soon) Lamp Circuit:

CIRCUIT DESCRIPTION
The On-Board Diagnostic System Check is an organized approach to identifying a problem created by an electronic engine control system malfunction. It must be the starting point for any driveability complaint diagnosis, because it directs the service technician to the next logical step in diagnosing the complaint. Understanding the chart and using it correctly will reduce diagnostic time and prevent the unnecessary replacement of good parts.

TEST DESCRIPTION
The numbers below refer to circled numbers on the diagnostic chart.

1. This step is a check for the proper operation of the Malfunction Indicator Lamp (MIL), (Service Engine Soon). The MIL should be "ON" steady.

2. Use Tech 1 to aid diagnosis, therefore, serial data must be available. If a PROM (MEM-CAL) error is present, the PCM may have been able to flash Diagnostic Trouble Code (DTC) 12/51, but not enable serial data.

3. Although the PCM is powered up, a "Cranks But Will Not Run" symptom could exist because of a PCM or system problem.

4. This step will isolate if the customer complaint is a MIL or a driveability problem with no MIL. Refer to "Diagnostic Trouble Code Identification" in this section for a list of valid DTCs. An invalid DTC may be the result of a faulty scan tool, PROM (MEM-CAL) or PCM.

5. Comparison of actual control system data with the Typical Tech 1 Data Values is a quick check to determine if any parameter is not within limits. Keep in mind that a base engine problem (i.e., advanced cam timing) may substantially alter sensor values.