Diagnostic Circuit Check

OBD Chart:

OBD Circuit:

The diagnostic circuit check is an organized approach to identifying a problem created by a control module 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.

Chart Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.

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

2. No MIL at this point indicates that there is a problem with the MIL circuit or the ECM control of that circuit.

3. This test checks the ability of the ECM to control the MIL. With the diagnostic terminal grounded, the MIL should flash a DTC 12 three times, followed by any DTC stored in memory. Depending upon the type of ECM or PROM (CALPAK) error may result in the inability to flash DTC 12.

4. This test checks the "Quad-Driver" circuit. If the MIL flashes less than three times, there is a problem with the MIL circuit or the ECM control of that circuit. Refer to "ECM QDR Check Procedure." ECM QDR Check Procedure

5. By using Tech 1 to aid diagnosis, serial data must be available. If a PROM (CAL-PAK) error is present, the ECM may have been able to flash DTC 12/51, but not enable serial data.

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

7. This step will determine if the customer complaint is a MIL or a driveability problem with no MIL. Refer to "TROUBLE CODE DESCRIPTION" Diagnostic Trouble Code Descriptions in this section for a list of valid DTC(s). An invalid DTC may be the result of a faulty scan tool, PROM or ECM.

8. Comparison of actual control system data with the typical scan 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.