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.

Effective control of exhaust emissions is achieved by an oxygen feedback system. The most important element of feedback system is O2S. The O2S is located in exhaust path. Once it reaches operating temperatures of 300-350°C (572-662°F), sensor generates a voltage that is inversely proportional to amount of oxygen in exhaust. The information obtained by sensor is used to calculate fuel injector pulse width. This maintains a 14.7:1 Air Fuel (A/F) ratio. At this mixture ratio, catalyst works best to remove hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx) from exhaust.

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 O2S circuit is monitored for a drop in voltage. The sensor output is used to test heater by isolating effect of heater element on O2S output voltage from other effects.