Diagnostic Circuit 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 and engine "CRANKS BUT WON'T RUN".
- Steady "CHECK ENGINE" light with the ignition "ON" and engine not running confirms battery voltage to the ECM.
- Code 12 should flash 3 times, followed by any other trouble codes stored in memory.
- Record all stored trouble code except code 12.
- With the engine running and the diagnostic terminal grounded, the ECM will respond to the O2 sensor signal voltage and use the "CHECK ENGINE" light to display this information as follows:
- A) Closed loop confirms that O2 sensor signal is being used by the ECM to control fuel delivery, and that the system is working normally. Signal voltage will vary from below .35 to above .55 volts.
- B) Open loop indicates that O2 sensor voltage is unusable to the ECM. Signal voltage is a constant value between .35 and .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 to closed loop, see Code 13.
- C) "CHECK ENGINE" light out indicates lean exhaust. O2 sensor signal voltage will be less than .35 volts and steady.
- D) "CHECK ENGINE" light on steady indicates rich exhaust. O2 sensor signal voltage will be greater than .55 volts and steady.
- Road test of the system using field service mode must be done at steady speeds. In this mode, the following conditions may be observed and are considered normal: Light on too long under acceleration, light off too long under deceleration, light on too long with idle below 1200 RPM.