- Powertrain/On-Board Diagnostic System Check

The Diagnostic Circuit check is an organized approach for identifying a problem caused by the Fuel Injection System.
Driver comments normally fall into one of the following areas:
^ Steady "CHECK ENGINE" light.
^ Driveability Problem.
^ Engine "Cranks But Will Not Run."

Understanding the chart and using it correctly will reduce diagnosis time and prevent the unnecessary replacement of parts.
1. A steady "CHECK ENGINE" light with the ignition "ON" and the engine stopped confirms battery and ignition voltage to the Electronic Control Module (ECM).
2. Ground diagnostic "test" terminal by jumpering terminal "A" to "B" in the ALCL connector located below the instrument panel.
The ECM will cause the "CHECK ENGINE" light to flash Code 12, indicating that the ECM diagnostics are working. Code 12 will flash three (3) times, followed by any other trouble codes stored in the memory. Each additional code will flash three (3) times, starting with the lowest code and then start over again with code 12. If there are no other codes, Code 12 will flash until the diagnostic "test" terminal jumper is disconnected or the engine is started.
3. Record all stored codes except for Code 12. If the problem is "Engine Cranks But Will Not Run", proceed to Chart A-3. Chart A-3A & A-3B Engine Cranks But Won't Run
4. If no additional codes were recorded, see Diagnosis by Symptoms for driveability symptoms and recommended service procedures. Depending on the severity of the problem, the "Field Service Mode" may be helpful in diagnosis. Testing and Inspection
With the engine running and the diagnostic terminal grounded, the ECM will respond to the Oxygen sensor signal voltage and use the "CHECK ENGINE" light to display this information as follows:
A. Closed loop confirms that the Oxygen sensor signal is being used by the ECM to control fuel delivery, and that the system is working normally.
Signal voltage will swing quickly from below 0.35 to above 0.55 volts.
B. Open loop indicates that the oxygen sensor voltage signal is not usable to the ECM. Signal voltage is at a constant value between 0.35 and 0.55 volts.
System will flash "open loop" from 30 seconds to 2 minutes after engine starts or until sensor reaches normal operating temperature. If system fails to go "closed loop", see Code 13. DTC 13
C. "CHECK ENGINE" light "OUT" indicates that exhaust is lean. O2 sensor signal voltage will be less than 0.35 volts and steady. See Code 44. DTC 44
D. "CHECK ENGINE" light on steady indicates that exhaust is rich. Sensor signal voltage will be above 0.55 volts and steady. See Code 45. DTC 45
5. Road test of the system using the field service mode should be done only at steady road speeds. Because the vehicle operates differently in the field service mode, the following conditions may be observed and should be considered normal.
^ Acceleration - Light may be "ON" too long due to acceleration enrichment.
^ Deceleration Light may be "OFF" too long due to decel enleanment or fuel cut-off.
^ Idle-Light may be "ON" too long with idle below 1200 RPM.
^ Surge 35 MPH timing changes from 10° to normal spark advance at 1600-1800 RPM.
6. The codes are cleared by disconnecting the ECM battery pigtail in the engine compartment for at least 10 seconds.