Theory Of Operation

The 4-Wire Oxygen 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 Sensor's 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 4-Wire Downstream O2 Sensor serves two functions. The first function is to measure the catalyst efficiency. If the Catalytic Converter is working properly, the oxygen content of the exhaust gases at the converter outlet fluctuate significantly less than the converter inlet. The PCM compares the switching rates of both Downstream and Upstream O2 Sensors under specific operating conditions to determine if the catalyst is functioning properly. Anytime the upstream to downstream switching ratio exceeds a calibrated value, a catalyst efficiency DTC is set.

The second function of the Downstream O2 Sensor is downstream fuel control and is active only during downstream closed loop operation. This function trims the 4-Wire upstream O2 Sensor goal voltage within the range of Upstream O2 Sensor operation. The Upstream O2 Sensor goal voltage is modified to ensure long Catalytic Converter life by allowing the PCM to control the amount of oxygen that is supplied to the catalytic converter. The downstream O2 Sensor is a good indicator of how much oxygen remains after all of the reactions take place in the converter.

If the O2 Sensor is powered up by the PCM and operating normally, the voltage readings should be as follows;

• Key on engine off.
  • Measured voltage on the Signal circuit with the sensor at ambient temperature should be near 35mv.
  • Measured voltage on the Return circuit with the sensor at ambient temperature should be near 15mv.

• Key on engine running.
  • Measured voltage on the Fused ASD Output circuit (at the PCM) should be the same as battery voltage.
  • Measured voltage on the Heater Control circuit should be near 7 volts.