E042

TEST CONDITIONS
1. Codes EO14, EO15, EO16, EO21, EO22, EO26, EO27, EO31, EO32 and EO34 not set;
2. Low battery condition does not exist;
3. Throttle switch open;
4. TPS equal to or greater than 6°;
5. TPS less than or equal to 30°;
6. Coolant temp equal to or greater than 58°C;
7. Left oxygen sensor ready;
8. RPM equal to or greater than 800;
9. TPS equal to or greater than 10° and canister purge has purged at full duty cycle for 5 minutes since the engine was running with coolant equal to or greater than 80°C.

FAILURE CONDITIONS
Left oxygen sensor remains lean for more than 50 seconds.

ACTION TAKEN
1. ECM turns on "Engine Control System Fault" telltale;
2. ECM commands BCM to display "Service Now" message on Driver Information Center (DIC);
3. ECM enables canister purge;
4. ECM disables AIR management solenoids;
5. ECM uses right oxygen sensor information for fuel control unless codes EOl3, EO44, or EO45 are set.

DESCRIPTION
The ECM provides a 0.45 volt reference signal to the oxygen sensor on CKT 412. When the oxygen sensor is cold (below 200°C), the oxygen sensor signal voltage will be around 0.45 volts and the ECM will keep the system in open loop operation. When the oxygen sensor is warm (above 200°C), the oxygen sensor will swing from rich to lean rapidly, at least one swing every two seconds, if the ECM is in good control of the air fuel mixture.
When the ECM sees that the oxygen is varying from the cold voltage of 0.45 volt, it will send the system into closed loop operation. In closed loop operation, the ECM will adjust the fuel delivery rate to the engine based on the oxygen sensor readings.

Code EO42 is designed so that if the oxygen sensor stays at a lean voltage for more than 50 seconds during the test conditions, Code EO42 will set.

Code EO42 will set when:
1. There is an oxygen sensor circuit fault giving a false lean indication
or
2. When the air fuel ratio is actually lean due to a vacuum leak or fuel control system fault.

Notes on Fault Tree
1. With the oxygen sensor disconnected, parameter ED32 should remain at reference voltage (0.38 to 0.63 volts).
2. Checking for sensor circuitry able to record rich readings. The DVOM set on volts will supply a very small current source to drive CKT 412 above 0.64 volts (rich). Similar results may be obtained by placing one finger on battery positive and another finger on oxygen sensor CKT 412 harness terminal.
3. The ECM compares oxygen sensor signal voltage received on circuit 412 to the ground voltage on circuit 413. If the ground voltage on circuit 413 is not engine ground (battery negative), the oxygen sensor can a appear falsely high or low. With the engine running, use a voltmeter to measure voltage from the oxygen sensor pin A to the ECM terminal B6. If the voltage is greater than -0.05 to +0.05 volts, repair poor ground on CKT 413 between ECM terminal B6 and engine ground.

Notes on Intermittents
Engine running, manipulate the oxygen sensor and ECM wiring and connectors while observing ECM Data ED32. If the fault is induced, ED07 will jump above 0.63 volts and the "OFF" status light will go on. Manipulate CKT 413 ground to the engine and look for a loose ground eyelet or ground eyelet installed at wrong location. Check oxygen sensor connector terminals for corrosion or foreign material. Clean if signs of corrosion are evident. If rich engine operation is suspected, perfor "PFI System Check." See Symptoms - "Poor Fuel Economy" and "Excessive Exhaust Emissions". Testing and Inspection
If the oxygen sensor circuit appears to be OK, go to PFI Chart A-9, "Oxygen Sensor Diagnosis" to check for faulty or contaminated sensor. Chart A-9 Left Oxygen Sensor Lean Exhaust Signal