E042

Diagnostic Chart.:

PCM Wiring:

DESCRIPTION:

The PCM/ECM provides a .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 PCM/ECM will keep the system in open loop operation. When the oxygen sensor is warm (above 200°C), the oxygen sensor signal voltage will swing from rich to lean rapidly, at least one swing every two seconds, if the PCM/ECM is in good control of the air fuel mixture. Code E042 is designed so that if the oxygen sensor stays at a lean voltage for more than 50 seconds during the test conditions, code E042 will set.

Code E042 will set when:

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

TEST CONDITIONS:

1. Codes E014, E015, E016, E021, E022, E026, E027, E031, E032 and E034 all MUST be clear.
2. Throttle switch open.
3. TPS between 6 and 30 degrees and steady.
4. Coolant sensor 58°C or greater.
5. Oxygen sensor ready (closed loop).
6. Engine speed greater than or equal to 800 RPM.
7. Canister purge has purged at full duty cycle for 10 minutes since the engine was running with coolant 80°C and TPS 10° or greater.

FAILURE CONDITIONS:

Oxygen sensor status stays lean for more than 50 seconds.

ACTION TAKEN:

1. PCM/ECM turns on "SERVICE ENGINE SOON" telltale.
2. PCM/ECM enables canister purge.

NOTES ON FAULT TREE:

1. With the oxygen sensor disconnected, parameter ED33 should remain at reference voltage (.38 to .63 volts)

2. Checking for sensor circuitry able to record rich readings. The DVOM set on volts will provide a few billionths of an amp to drive CKT 912 to above .64 volts (rich). Similar results may be obtained by placing one finger on battery positive terminal and another finger on oxygen sensor CKT 912 harness terminal.

3. The PCM/ECM compares oxygen sensor signal voltage received on circuit 912 to the ground voltage on circuit 413. If the PCM/ECM doesn't have a good ground to the engine on circuit 413, the oxygen sensor can appear falsely high or low. With engine running, use a voltmeter to measure voltage from the oxygen sensor pin A at the exhaust manifold to the PCM/ECM terminal E15. If the voltage is greater than -.05 volts to +.05 volts then repair poor ground on CKT 413 between PCM/ECM terminal E15 and the ground at front of engine, right (rearmost) head.

NOTE ON INTERMITTENTS:

Engine running, manipulate the oxygen sensor and PCM/ECM wiring and connectors while observing PCM/ECM data ED32. If the fault is induced, ED32 will jump below .37 volts and the "ECON" status light will go off. 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 signs of corrosion or connector terminals for signs of corrosion or foreign material. Clean if signs of corrosion are evident. If lean engine operation is suspected, perform PFI SYSTEM CHECK under "A" CHARTS. An engine that is misfiring may have a lean oxygen sensor indication because of unburned oxygen in the exhaust stream. If the oxygen sensor circuit appears to be OK, go to PFI Chart A-4, oxygen sensor diagnosis, to check for faulty or contaminated sensor.