ECM Code EO42: Left O2 Sensor Lean 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. 412. When oxygen sensor is cold, less than 200°C, oxygen sensor signal voltage will be about .45 volt. The ECM will keep system in open loop operation. When oxygen sensor is warm (greater than 200°C) oxygen sensor signal voltage will change from rich to lean rapidly (at least one change every 2 seconds) if ECM is in good control of air/fuel mixture.

When ECM sees that oxygen sensor is varying from cold voltage of .45-volt, 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 will set if oxygen sensor stays at lean voltage for more than 50 seconds during test conditions. Code EO42 will set when there is an oxygen sensor circuit fault giving a false lean indication, or when air/fuel ratio is actually lean due to a vacuum leak or fuel control system fault.

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

Test Number 1) With oxygen sensor disconnected, parameter ED07 should remain at reference voltage (.38-.63 volt).

Test Number 2) Checks for sensor circuitry ability to record rich readings. The DVOM set on volts will provide a few billionths of an amp to drive circuit No. 912 to greater than .64 volt (rich). Similar results may be obtained by placing a finger on battery positive terminal and another finger on oxygen sensor circuit No. 912 harness terminal.

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 drop 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.