Action Taken
- ECM turns on "ENGINE CONTROL SYSTEM FAULT" light.
- ECM commands BCM to display "SERVICE NOW" message on DIC.
- ECM turns off canister purge and air management solenoids.
- ECM uses right O2 sensor reading for fuel adjustments unless Codes EO13, EO44 or EO45 are set.
The ECM provides a .45-volt reference signal to oxygen sensor on circuit No. 412. When oxygen sensor temperature is below 392°F (200°C), oxygen sensor signal voltage will be about .45 volts and ECM will keep system in open loop operation. When oxygen sensor temperature is above 392°F (200°C), oxygen sensor signal voltage will swing from rich to lean rapidly. At least one swing every 2 seconds will occur if ECM is in good control of air-fuel mixture.
When ECM sees that oxygen sensor signal is varying from cold voltage of .45-volts, it will send system into closed loop operation. In closed loop operation, ECM will adjust fuel delivery rate to engine based on oxygen sensor readings.
Code EO42 is set when left oxygen sensor stays at lean voltage for more than 50 seconds during test conditions. Code E042 will also set when:
- There is an oxygen sensor circuit fault giving a false lean indication.
- When air-fuel ratio is actually lean due to a vacuum leak or fuel control system fault.
- With oxygen sensor disconnected, parameter ED32 should remain at reference voltage (.38-.63 volts).
- Checks if sensor circuitry able to record rich readings. The DVOM set on volts will provide a few billionths of an amp to drive circuit No. 412 to above .64 volts (rich). Similar results may be obtained by placing a finger on positive post of battery and another finger on oxygen sensor circuit No. 412 harness terminal .
- The ECM compares oxygen sensor signal voltage received on circuit No. 412 to ground voltage on circuit No. 413. If ECM does not have a good ground to engine on circuit No. 413, oxygen sensor can appear falsely high or low.
With engine running, use voltmeter to measure voltage from oxygen sensor at exhaust manifold to ECM terminal "A". If voltage is not within the range of -.05 to +.05 volts, repair poor ground on circuit No. 413 between ECM terminal "B6" and engine ground.