Diagnostic Circuit Check: Notes

The diagnostic circuit check is an organized approach for identifying a problem caused by the fuel injection system. Driver complaints fall into 3 categories: Steady "CHECK ENGINE" light, driveability problems and "ENGINE CRANKS BUT WILL NOT RUN". Understanding chart and using it correctly will reduce diagnosis time and prevent unnecessary replacement of parts.

1. A steady "CHECK ENGINE" light with ignition on and engine not running confirms battery and ignition voltage to Electronic Control Module (ECM).
2. Code 12 will flash 3 times, followed by other trouble codes stored in memory.
3. Record all stored codes, except Code 12. If the problem is "ENGINE CRANKS BUT WILL NOT RUN", proceed to Chart A-3.
4. If no additional codes were recorded, see trouble shooting information in the CCC TESTS W/O CODES article in this section for driveability symptoms and recommended service procedures. With engine running and diagnostic terminal grounded, the ECM will respond to the oxygen sensor signal and use the "CHECK ENGINE" light to display the following information:
   • A) Closed loop confirms that oxygen sensor signal is being used by the ECM to control fuel delivery and that system is working properly. Signal voltage will vary from below .35 to above .55 volts.
   • B) Open loop indicates that oxygen sensor signal is not usable to ECM. Signal voltage will be constant and between .35 and .55 volts. System will flash "open loop" for 30 seconds to 2 minutes after engine starts or until sensor reaches normal operating temperature. If system fails to go into closed loop, see Code 13.
   • C) "CHECK ENGINE" light "OFF" indicates that exhaust is lean. Oxygen sensor will be less than .35 volts and steady. See Code 44.
   • D) "CHECK ENGINE" light on steady indicates that exhaust is rich. Oxygen sensor signal will be above .55 volts and steady. See Code 45.
5. Road test of the system in the field service mode should be done only at steady road speeds. The following conditions may be observed and should be considered normal: Light "ON" too long under acceleration, light "OFF" to long under deceleration or light "ON" too long with idle below 1200 RPM.