Theory Of Operation

The 4-Wire Oxygen (O2) Sensor is equipped with a galvanic battery that typically generates a voltage signal between 0.0 volts and 1.0 volts. The 4-Wire O2 Sensor also includes a heating element that keeps the sensor at proper operating temperature during all operating modes. Maintaining correct sensor temperature at all times allows the system to enter into closed loop operation sooner. Also, it allows the system to remain in closed loop operation during periods of extended idle.

A bias voltage in the Powertrain Control Module (PCM) shifts the signal voltage to fluctuate between 2.5 volts and 3.5 volts, depending upon the oxygen content of the exhaust gas. When a large amount of oxygen is present (caused by a lean air/fuel mixture) the sensor produces a low voltage. When there is a lesser amount of oxygen present (caused by a rich air/fuel mixture) it produces a higher voltage. By monitoring the oxygen content and converting it to electrical voltage, the sensor acts as a rich-lean switch.

In Open Loop operation the PCM ignores input from the O2 Sensors. In Closed Loop operation the PCM monitors the O2 Sensors input (along with other inputs) and adjusts the injector pulse width accordingly based on pre-programmed (fixed) values and inputs from other sensors. The PCM also compares upstream and downstream O2 Sensor inputs to calculate the catalytic convertor oxygen storage capacity and converter efficiency.

The circuit fault diagnostics for O2 Sensors are algorithm based. This means that the fault does not necessarily set the same as a traditional hard circuit fault such as a short or an open. The PCM determines O2 Sensor circuit faults based on pre-programmed variables that are used in a calculation to determine the state of an O2 Sensor. This requires the integrity of all circuits to be verified in order to ensure that the cause of the fault is not overlooked.