PFI Chart A-9: Left O2 Sensor Lean: Description
The ECM provides a .45-volt reference signal to oxygen sensor on circuit No. 412. When warm, a properly operating oxygen sensor will drive the reference voltage lower (below .45 volts) to indicate a lean mixture and higher (above .45 volts), to indicate a rich mixture.
Fixed low oxygen sensor voltage readings are normally evidence that the air fuel mixture is lean and the closed loop system is unable to compensate sufficiently due to some malfunction in the engine emission or fuel system. Less likely is the possibility that the sensor has failed and is giving an incorrect low reading. If this is the case, the closed loop fuel system will be over compensating and causing rich operation (black smoke, fouled plugs, poor fuel economy, high HC and CO, etc.) while the O2 sensor is indicating lean.
- If code EO17 is stored do not use this chart, go to PFI CHART A-7: LEFT O2 SENSOR NOT READY .
- With the sensor disconnected, the oxygen sensor reference voltage (.38 to .63 volts) should appear. If the ED32 reading is less than .38 volts then check for grounded signal CKT 412 or open ground CKT 413.
- 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 412 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 412, harness terminal.
- The ECM compares oxygen sensor signal voltage received on CKT 412 to the ground voltage on CKT 413. If the ECM doesn't have a good ground to the engine on CKT 413, the oxygen sensor can appear falsely high or low. With engine running, use a voltmeter to measure voltage from the oxygen sensor at the exhaust manifold to Pin B6 at the ECM. If the voltage is -.05 volts to +.05 volts then the ground is OK. If the voltage is less than -.05 volts or greater than +.05 volts, repair poor connection between ECM Pin B6 and oxygen sensor ground eyelet on front of right or rear engine cylinder head.
- If the oxygen sensor circuit is OK, then Lean Exhaust is caused by Fuel/Emissions Systems not controlling.
- A fuel delivery system which is not functioning properly may cause a lean fuel mixture. This malfunction can be caused by fuel pressure less than 40 PSI at the injectors, by defective injectors, etc. Refer to PFI CHART A-4A: STATIC FUEL PRESSURE DIAGNOSIS for additional information.
- If the "Left Oxygen Sensor Test" (PFI CHART A-13: LEFT O2 SENSOR DIAGNOSIS ) indicates that the sensor is OK, then check all connectors, terminals, etc. for an intermittent condition.
- Check for vacuum leaks at hoses, intake manifold and throttle body gaskets.
- Check for intermittent injector operation, see PFI CHART A-5: INJECTOR SYSTEM DIAGNOSIS .