Oxygen Sensor Heater Monitor — I/M Oxygen Sensor Heater Monitor

Oxygen Sensor Heater Monitor

The Oxygen Sensor Heater Monitor’s job is to constantly monitor the operation of the oxygen sensor’s heater.

There are two modes of operation on a computer-controlled vehicle: “open-loop” and “closed-loop.”

The vehicle operates in open-loop when the engine is cold. During open loop, the ECM calculates fuel based on outside air temperature and engine coolant temperature sensor readings only, ignoring data from the oxygen sensors. However, once the oxygen sensors’ heaters bring the sensor up to 600°F operating temperature, the ECM switches to closed loop operation and begins factoring in actual oxygen sensor readings to calculate fuel.

When the oxygen sensor monitor sets a trouble code

The oxygen sensor heater receives battery voltage from a fuse and the ground portion of the circuit terminates through the ECM. Based on the voltage drop on the ground side, the ECM can determine the state of the oxygen sensor heater. If the return voltage is out of range, the ECM will set an oxygen sensor heater related trouble code and turn on the check engine light.

The Oxygen Sensor Heater Monitor is a “Two-Trip” Monitor. If a fault is found on the first trip, the computer temporarily saves the fault in its memory as a Pending Code. The computer does
not command the MIL on at this time. If the fault is sensed again on the second trip, the computer commands the MIL “On,” and saves the code in its long-term memory.

©, 2022 Rick Muscoplat