Diagnostic System Check - With OBD Scanner

THE CORRECT PROCEDURE TO DIAGNOSE A FAULT IS TO FOLLOW THREE BASIC STEPS:

IMPORTANT:
After the visual/physical underhood inspection, the On-Board Diagnostic (OBD) system check is the starting point for all diagnostic procedures and for locating the cause of an emissions test failure.

1. Are the On-Board Diagnostics Working?
This is determined by performing the OBD system check. This is the starting point for the diagnostic procedures and for finding the cause of an emissions test failure. If the on-board diagnostics are not working, the OBD system check will lead to diagnostic charts. If the on-board diagnostics are working properly, the next step will be:

2. Is there a Diagnostic Trouble Code (DTC) Stored?
If a DTC is stored, go directly to the numbered DTC chart in the applicable subsection. This will determine if the fault is still present. If no DTC is stored, the next step will be:

3. Scan Tool Serial Data Transmitted by the Control Module.
This involves displaying the information available on the serial data stream with a TECH 1 scan tool or one of the tools available for that purpose. Information on these tools and the meaning of the various displays can be found in the succeeding paragraphs. Normal readings under a particular operating condition can be found in the OBD system check.

ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK

On-Board Diagnostic System Check:

On-Board Diagnostic Circuit:

CIRCUIT DESCRIPTION:
The On-Board Diagnostic System Check is an organized approach to identifying a problem created by a control module system malfunction. Understanding the chart and using it correctly will reduce diagnostic time and prevent the unnecessary replacement of good parts.

CHART TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. This step is a check for the proper operation of the MIL (Service Engine Soon). The MIL should be "ON" steady.
2. No MIL at this point indicates that there is a problem with the MIL circuit or the Powertrain Control Module (PCM) control of that circuit.
3. This test checks the ability of the PCM to control the MIL. With the diagnostic terminal grounded, the MIL should flash a DTC 12 three times, followed by any DTCs stored in memory. Depending upon the type of PCM or PROM (MEM-CAL) error may result in the inability to flash DTC 12.
4. Use TECH 1 scan tool 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 DTC 12/51, but not enable serial data.
5. Although the PCM is powered up, a "Cranks But Will Not Run" symptom could exist because of a PCM or system problem.
6. This step will isolate if the customer complaint is an MIL or a driveability problem with no MIL. For a list of valid DTCs, refer to Trouble Code Description.
Diagnostic Trouble Code Descriptions
An invalid DTC may be the result of a faulty scan tool, PROM (MEM-CAL) or PCM.
7. 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.