Pcm Code EO43, Left O2 Sensor Rich Signal: Action:

PCM turns on SERVICE ENGINE SOON light. PCM turns on canister purge solenoid.

The PCM provides a .45-volt reference signal to oxygen sensor on circuit No. 912. When oxygen sensor is cold, less than 392°F (200°C), output voltage will be about .45 volt and PCM will keep system in open loop operation. When warm, a properly operating oxygen sensor will drive the .45-volt reference higher (greater than .45 volt) to indicate a rich mixture. Oxygen sensor signal voltage will swing from rich to lean rapidly. This will produce at least one swing every 2 seconds, if PCM is in good control of air/fuel mixture.

When PCM sees that oxygen sensor is not at cold voltage of .45 volt, it will send system into closed loop operation. In closed loop operation, PCM will meter fuel based on oxygen sensor readings.

Code EO43 will set if oxygen sensor stays at rich voltage more than 45 seconds during test conditions. Code EO43 will set when oxygen sensor circuit fails, or when air/fuel ratio is actually rich due to fuel control or emissions system fault.

NOTE: Test numbers refer to test numbers on diagnostic chart.

With oxygen sensor disconnected, parameter ED33 should remain at reference voltage (.38-.63 volt).
This checks oxygen sensor circuitry to record lean readings.
The PCM compares oxygen sensor signal voltage received on circuit No. 912 to ground voltage on circuit No. 413. If PCM does not have a good ground to engine on circuit No. 413, oxygen sensor can appear falsely high or low. With engine running, use a voltmeter to measure voltage between oxygen sensor at exhaust manifold and PCM terminal B6. If the voltage is -.05 to +.05 volt, ground is okay. If voltage is less than -.05 volt or greater than +.05 volt, repair poor ground on circuit No. 413 between PCM terminal B6 and ground at front of engine on right cylinder head.