Oxygen Sensor Heater Monitor

If the Oxygen Sensor (O2S) DTC as well as a O2S heater DTC is present, the O2S Heater DTC MUST be repaired first. After the O2S Heater is repaired, verify that the sensor circuit is operating correctly.

NOTE: The O2S Heaters are kept off at coolant temperatures below 20° C (68° F) and at high engine rpm in order to avoid damaging the heaters. The voltage reading taken from the O2S are very temperature sensitive. The readings taken from the O2S are not accurate below 300° C (572° F). Heating the O2S is done to allow the engine controller to shift to closed loop control as soon as possible. The heating element used to heat the O2S must be tested to ensure that it is heating the sensor properly. The heater resistance is checked by the PCM almost immediately after the engine is started. The same O2S heater return pin used to read the heater resistance is capable of detecting an open, shorted high or shorted low circuit.

The Oxygen Sensor Heater Monitor begins after the ignition has been turned OFF and the O2 sensors have cooled. As the sensor cools down, the resistance increases and the PCM reads the increase in voltage. Once voltage has increased to a predetermined amount, higher than when the test started, the oxygen sensor is cool enough to test heater operation.

When the oxygen sensor is cool enough, the PCM provides a ground path for the O2S heater circuit. Voltage to the O2 sensor begins to increase the temperature. As the sensor temperature increases, the internal resistance decreases.

The heater elements are tested each time the engine is turned OFF if all the enabling conditions are met. If the monitor fails, the PCM stores a maturing fault and a Freeze Frame is entered. If two consecutive tests fail, a DTC is stored. Because the ignition is OFF, the MIL is illuminated at the beginning of the next key cycle, after the 2nd failure.