Diagnostic Circuit Check
- If complaint is "CHECK ENGINE" lamp related, this check will lead to the most likely problem area, if a malfunction exists. Enter diagnostic mode and record stored trouble codes. Begin diagnosis with the lowest numbered code shown and go to the numbered trouble code chart.
- If code "51" is displayed, see PROM removal and installation in this article. If codes "54" or "55" are displayed with another code, always refer to diagnostic chart for code "54" or "55" first, then proceed to next lowest numbered code.
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 "SERVICE ENGINE SOON" light, driveability problems, and engine "CRANKS BUT WON'T RUN."
NOTE:
The following step numbers refer to the numbers in the accompanying flow chart(s).
- 1) A steady "SERVICE ENGINE SOON" light, with the ignition "ON" and engine stopped, confirms battery and ignition voltage to the ECM.
- 2) Code 12 should flash 3 times, followed by any other trouble codes stored in memory.
- 3) Record all stored codes except Code 12.
- 4) With the engine running and the diagnostic terminal grounded, the ECM will respond to the O2 sensor signal voltage and use the "SERVICE ENGINE SOON" light to display this information as follows:
- A) Closed loop confirms that O2 sensor voltage is being used to control fuel delivery. Signal voltage will vary from .35-.55 volt.
- B) Open loop confirms that O2 sensor voltage is unusable to the ECM. Signal voltage is a fixed value between .35-.55 volt. System will flash open loop for 30 seconds to 2 minutes or until O2 sensor reaches operating temperature.
- C) O2 sensor signal voltage will be less than .35 volt.
- D) O2 sensor signal voltage will be more than .55 volt.
- 5) Road test of the system in the field service mode must be done at steady speeds. In this mode the following conditions may be observed and considered normal: Light "ON" too long under acceleration, light "OFF" too long under deceleration, light on too long at idle with idle below 1200 RPM.