Oxygen Sensor Heater Monitor

If there is an oxygen sensor (O2S) DTC as well as a O2S heater DTC, O2S fault MUST be repaired first. After O2S fault is repaired, verify that heater circuit is operating correctly.

The voltage readings taken from O2S are very temperature sensitive. The readings are not accurate below 300°C. Heating of O2S is done to allow engine controller to shift to closed loop control as soon as possible. The heating element used to heat O2S must be tested to ensure that it is heating sensor properly.

The heater element itself is not tested. The sensor output is used to test heater by isolating effect of heater element on O2S output voltage from other effects. The resistance is normally between 100 ohms and 4.5 megohms. When oxygen sensor temperature increases, resistance in internal circuit decreases. The PCM sends a 5 volt biased signal through oxygen sensors to ground this monitoring circuit. As temperature increases, resistance decreases and PCM detects a lower voltage at reference signal. Inversely, as temperature decreases, resistance increases and PCM detects a higher voltage at reference signal. The O2S circuit is monitored for a drop in voltage.