ECM Code EO43: Left O2 Sensor Rich Signal: Action:

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

The ECM provides a .45-volt reference signal to oxygen sensor on circuit No. 912. When oxygen sensor is cold (less than 200°C), output voltage will be about .45 volt and ECM 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 ECM is in good control of air/fuel mixture.

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

Code EO43 will set if oxygen sensor stays at rich voltage for 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.

Test Number 1) With oxygen sensor disconnected, parameter ED33 should remain at reference voltage (.38-.63 volt).

Test Number 2) This checks ECM's ability to recognize lean input on oxygen sensor signal circuit No. 912.

Test Number 3) The ECM compares oxygen sensor signal voltage received on circuit No. 912 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 a voltmeter to measure voltage between oxygen sensor at exhaust manifold and ECM terminal B6. If 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 ECM terminal B6 and ground at front of engine on right cylinder head.