Diagnostic System Check - Engine Codes

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 On-Board Diagnostic (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 On-Board Diagnostic (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 On-Board Diagnostic (OBD) system check.

OBD Check Chart:

OBD Circuit:

ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK

CIRCUIT DESCRIPTION:
The on-board diagnostic system 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 MIL (Service Engine Soon). The MIL should be "ON" steady.
2. Use TECH 1 scan tool to aid diagnosis, therefore, serial data must be available. If a PROM (MEM-CAL) error is present, the Powertrain Control Module (PCM) may have been able to flash 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. For a list of valid DTCs, refer to DIAGNOSTIC TROUBLE CODE DESCRIPTION
Diagnostic Trouble Code Descriptions
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.