OBD II Readiness Monitors

Non-Continuous Monitors

Oxygen Sensor Heater Monitor

OBD II equipped vehicles use heated oxygen sensors. Heated oxygen sensors have an internal heater circuit that brings the sensor up to operating temperature more quickly than an unheated sensor. The heater will bring the sensor up to operating temperature within 20 to 60 seconds depending on the sensor, and also keep the oxygen sensor hot even when the engine is idling for a long period of time. The faster the oxygen sensor heats up, the quicker the PCM can enter closed loop fuel control, optimizing catalytic converter efficiency.  

On most OBD II systems the oxygen sensor heater monitor runs concurrently with the oxygen sensor monitor or continuously when the engine is running. However, the successful completion of the monitors are independent events. Typically the PCM will enable the monitor, conditions for completing the oxygen sensor monitor will be met and that monitor will successfully complete. Some enabling criteria to complete the oxygen sensor heater monitor may not always occur and the heater monitor goes uncompleted for several drive cycles. This may happen more frequently as the sensors age and the sensors’ heating element deteriorates. 

During oxygen sensor heater monitoring the PCM turns the heater strip circuit on and off for each sensor and looks for an expected change in current. On some OBD II systems the PCM continuously monitors current flow through the heater circuit. The heater circuit of each oxygen sensor is evaluated independently.