Code P042, Front Oxygen Sensor Lean Signal: Notes

PCM provides a .45-volt reference signal to oxygen sensor on circuit No. 412. When oxygen sensor temperature is less than 392°F (200°C), sensor signal voltage will be about .45 volt and PCM will keep system in open loop operation. When oxygen sensor temperature is greater than 392°F (200°C), sensor will begin to generate a signal voltage. In a rich environment, signal voltage will be greater than .45 volt. In a lean environment, signal voltage will be less than .45 volt. If PCM is in good control of air/fuel mixture, mixture will change from rich to lean rapidly (at least one change every 2 seconds).

When PCM sees oxygen sensor is generating a signal voltage (greater than .45 volt), PCM will send system into closed loop operation. In closed loop operation, PCM will adjust fuel delivery rate to engine based on oxygen sensor readings. Code P042 sets if oxygen sensor stays at lean voltage more than 50 seconds during test conditions. Code P042 also sets when an oxygen sensor circuit fault giving a false lean indication exists or when air/fuel ratio is actually lean due to a vacuum leak or fuel control system fault.