Theory Of Operation

The primary objective of the Oxygen (O2) Sensor Heater Control is to attain the fastest possible temperature rise, and to control the steady state temperature of the O2 Sensor. A Positive Temperature Coefficient (PTC) element inside the O2 Sensor heats up as current passes through it. This allows the system to enter Closed Loop quickly and meet the OBD II requirements for continuous monitoring and to deliver accurate readings. This is accomplished by the Powertrain Control Module (PCM) sending a Pulse Width Modulated (PWM) current to the O2 Sensors heater element. The PWM duty cycle is calculated, in response to the temperature measurements. The PCM uses the collected information to continually adjust the PWM to achieve a desired steady state temperature target.

The heater temperature is obtained by measuring the heater resistance and calculating the heater temperature using the resistance versus temperature characteristic. In order to measure the heater temperature, the transistor that controls the heater is momentarily disabled and current is passed through a precision sense resistor connected in series with the heater element. The voltage drop across the sense resistor provides the relationship to current which equates to a temperature value. Once the temperature is acquired, the sense current is turned off and normal operation is restored. The actual heater temperature is monitored continuously and the heating rate is adjusted to prevent damaging the heating element.