Heated oxygen sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the vehicle is first started, the engine control module (ECM) operates in an Open Loop mode, ignoring the HO2S signal voltage when calculating the air-to-fuel ratio. The ECM supplies the HO2S with a reference, or bias, voltage of about 450 mV. The HO2S generates a voltage within a range of 0-1,000 mV that fluctuates above and below bias voltage. High HO2S voltage output indicates a rich fuel mixture. Low HO2S voltage output indicates a lean mixture. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature, and to provide an accurate voltage signal. Fuel trim biasing is used by the ECM to keep the post catalyst HO2S voltage as close to 600 mV as possible. This allows optimal catalyst efficiency under typical operating conditions. The ECM constantly monitors the count of fuel trim bias being commanded. If the ECM detects that rich fuel trim bias is commanded for longer than a calibrated amount of time, DTC P2097 will set for Bank 1, or DTC P2099 will set for Bank 2.

The ECM detects that the HO2S 1 1 fuel trim bias is commanded rich for longer than 1 second, within a 200 second monitoring period.