The oxygen sensors supply the PCM (computer) with a signal that indicates a rich or lean condition during engine operation. This input information assists the computer in determining the proper air/fuel ratio. A low voltage signal from one or more sensors indicates too much oxygen in the exhaust (lean condition) and, conversely, a high voltage signal indicates too little oxygen in the exhaust (rich condition). The oxygen sensors are threaded into the exhaust manifold and/or exhaust pipes on all vehicles. Heated oxygen sensors are used on all models to allow the engine to reach the closed loop faster. A faulty oxygen sensor due to loose connections, bad grounds, high resistance in the circuit, or opens in the circuit can cause the following symptoms.
The PCM sends a bias voltage of approximately 450 mV to the oxygen sensor. At operating temperature the oxygen sensor signal varies between 0 and 1,000 mV. When the mixture is rich the oxygen content is low and the voltage signal will remain on the high side of the 450 mV mid-range. When the mixture is lean the oxygen content is high and the voltage signal will remain on the low side below the 450 mV mid-range. The oxygen sensors on later model vehicles are equipped with a heater circuit. The heater circuit in the oxygen sensor shortens the time required for the sensor to reach operating temperature and provides a more accurate signal.
Do not pierce the wires when testing heated oxygen sensors, as this can lead to wiring harness damage. Backprobe the connector to properly read the voltage of HO2S.When testing the oxygen sensor voltage signal, it should correspond to the values shown in this chart
NOTE: If excessive force is needed to remove the sensors, lubricate them with penetrating oil prior to removal.