Diagnostic Circuit Check Procedures
- Steady "SERVICE ENGINE SOON" 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 codes (except Code 12).
- With the engine running and the diagnostic terminal grounded, the ECM will respond to the oxygen sensor signal voltage and use the "SERVICE ENGINE SOON" light to display this information as follows:
- Closed loop confirms the O2 sensor signal is being used by the ECM to control fuel delivery, and the system is working normally. Signal voltage will vary from below .35 to above .55 volts.
- Open loop indicates that oxygen 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.
- "SERVICE ENGINE SOON" light out indicates lean exhaust. Oxygen sensor signal voltage will be less than .35 volts and steady.
- "SERVICE ENGINE SOON" light on steady indicates rich exhaust. O2 sensor signal voltage will be greater than .55 volts and steady.
- Road test of the vehicle 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.
- Clearing codes. Ignition should be off. Remove continuous battery fuse for 30 seconds. Fuse and holder are located near battery.