CIRCUIT DESCRIPTION:
The ECM applies a bias voltage of approximately 450 millivolts (350-550 mV is normal bias voltage) between terminals "GEI4" and "GElS." The oxygen (02) sensor varies the voltage within a range of about 1 volt, if the exhaust is rich, down through about .10 volt, if exhaust is lean. As each pair of exhaust valves open, a different exhaust oxygen content passes by the oxygen sensor, which causes the oxygen sensor voltage output to vary. A lean exhaust condition will cause the oxygen sensor to output a low voltage, which will pull the bias voltage from CKT412 low. The ECM is programmed to interpret any voltage less than 500 mV as a "lean exhaust condition."The sensor is like an open circuit and produces no voltage when it is below about 360'C (680'F). An open sensor circuit causes "Open Loop" operation.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
- This test checks the oxygen sensor's heating element. The heating element resistance should be 3.5 ohms at 20'C (68'F) or 14 ohms at 350'C (562'F).
- Code 44 is set when the oxygen (02) sensor signal voltage on CKT 412;
- Remains below .2 volt for 2 minutes and the fuel system is operating in "Closed Loop."
DIAGNOSTIC AIDS:
Using the Tech I diagnostic computer "Scan" tool, observe the block learn values at different rpm and air flow conditions. The "Scan" tool also displays the block cells, so the block learn values can be checked in each of the cells to determine when the Code 44 may have been set. If the conditions for Code 44 or Code 64 exist, the block learn values will be around 150.
- O2 Sensor Wire. Sensor pigtail may be mispositioned and contacting the exhaust manifold.
- Check for intermittent ground in wire between connector and sensor.
- Poor connection at oxygen (02) sensor ground wire.
- Lean Injector(s). Perform injector balance test CHART C-2A.
- Fuel Contamination. Water, even in small amounts, near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a lean exhaust and can set a Code 44 or Code 64.
- Fuel Pressure. System will be lean if pressure is too low. It may be necessary to monitor fuel pressure while driving the car at various road speeds and/or loads to confirm. Refer to Fuel System Diagnosis CHART A-7.
- Exhaust Leaks. If there is an exhaust leak, the engine can cause outside air to be pulled into the exhaust and past the sensor. Vacuum or crankcase leaks can cause a lean condition.
- If the above are OK, it is a faulty oxygen sensor.