GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2000 🢒 Firebird 🢒 Body and Accessories 🢒 Wiring Systems 🢒 Powertrain On Board Diagnostic (OBD) System Check

Refer to Engine Controls Schematics .

Circuit Description

The Powertrain On-Board Diagnostic System Check must be the starting point for any driveability complaint diagnosis. Before using this procedure, you should perform a careful visual/physical inspection of the PCM and engine grounds for clean and secure connections. The Powertrain On-Board Diagnostic System Check is an organized approach to identifying a problem created by an electronic engine control system malfunction.

Diagnostic Aids

Inspect for the following:

Many situations may lead to an intermittent condition. Perform each inspection or test as directed.

Important: :  Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

    • Loose terminal connection
       -  Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.
       -  Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.
    • Damaged harness--Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.
    •  Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.

If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.

Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table:

  1. The MIL should be ON steady with the key ON, engine OFF. If not, the No Malfunction Indicator Lamp table should be used to isolate the malfunction.

    2.

  2. This test ensures that the PCM is capable of transmitting Class 2 serial data to the DLC and that the Class 2 data circuit is not open or shorted. If a problem is encountered and a malfunctioning scan tool is suspected, try the scan tool on another vehicle to verify operation. If a DLC problem exists, the DLC Diagnosis table should be used to diagnose the condition.

    3.

  3. If the engine will not start, the Engine Cranks But Will Not Run table should be used to diagnose the condition.

    4.

  4. The scan tool may display DTCs which are diagnosed in Automatic Transmission/Transaxle Diagnosis. If multiple DTCs are stored, diagnose each DTC according to the following priority:

    5. • PCM Error DTCs.
    • System Voltage DTCs.
    • Component Level DTCs (Switches, sensor range/performance, sensor high voltage, sensor low voltage, ODMs, etc.).
    • System level DTCs (Fuel trim, Misfire, EGR flow, TWC System, Idle Control System, HO2S Response or HO2S Transition Time Ratio).

  5. A scan tool parameter which is not within the typical range may help to isolate the area which is causing the problem.

Powertrain On--Board Diagnostic System Check (w/out start inh)

Step

Action

Value(s)

Yes

No

1

Important:: 

   • Check for applicable service bulletins before proceeding with the diagnosis.
   • Do not turn OFF the ignition when performing this diagnostic table.
   • Do not perform this test if no driveability condition exists.
   • Unless instructed, do not clear any DTCs.

  1. Turn ON the ignition, with the engine OFF.
  2. Observe the Malfunction Indicator Lamp (MIL).

Is the MIL illuminated?

--

Go to Step 2

Go to Malfunction Indicator Lamp (MIL) Inoperative

2

  1. Install a scan tool.
  2. With a scan tool, attempt to display PCM data.

Does the scan tool display PCM data?

--

Go to Step 3

Go to Data Link Connector Diagnosis

3

Attempt to start the engine.

Does the engine start and continue to run?

--

Go to Step 4

Go to Engine Cranks but Does Not Run

4

Important: If the scan tool indicates any DTCs set, record Freeze Frame/Failure Records.

Use the scan tool in order to display Diagnostic Trouble Codes (DTCs).

Does the scan tool indicate any DTCs set?

--

Go to the applicable DTC table

Go to Step 5

5

With a scan tool, compare the PCM data to the Scan Tool Data List .

Does the scan tool indicate PCM values equal or within the typical values?

--

System OK

Go to Component System Checks