Oxygen Sensor Heater Check

In order for the oxygen sensor to operate correctly the sensor element must be heated. An improperly/non operating heater will not allow the sensor signal to reach its predefined maximum and minimum thresholds which can result in delayed closed loop operation causing an impact on emission levels, or in increased emission levels while in closed loop operation. As part of the monitoring function for heater current and voltage, circuit is also checked for an open, short to ground and short to voltage depending on values of current or voltage being monitored.

On Bosch systems, heater monitoring function measures both sensor heater current and heater voltage in order to calculate sensor heater resistance and power. Oxygen sensor heater current is calculated via a voltage drop over a shunt resistor, internal to control module. If power of heater is not within a specified range, a fault will be set. The next time the heater circuit is monitored and a fault is again present, CHECK ENGINE light will be illuminated. Heater function is monitored continuously while vehicle is in closed loop operation, as long as the heater is activated by the Engine Control Module.

On Siemens systems, if heater output is too low, signal amplitude of oxygen sensor will be reduced. If predetermine minimum low (rich) voltage and high (lean) voltage can not be obtained within a predefined time, a fault will be set. Next time heater circuit is monitored and a fault is again present, CHECK ENGINE light will be illuminated. Heater function of pre-cat sensor is monitored continuously while the vehicle is in closed loop operation, as long as the heater is activated by the Engine Control Module.