Monitor Description
The vehicle is equipped with two oxygen sensors (O2S). One is mounted upstream from the three-way catalytic (TWC) converter (Front Oxygen Sensor, "sensor 1"), the second is mounted downstream (Rear Oxygen Sensor "sensor 2"). The catalyst efficiency monitor compares sensor 1 and sensor 2 signals in order to calculate TWC ability to store the oxygen.
During normal operation, the TWC stores and releases oxygen as needed. This results in low oxygen variations in the post TWC exhaust stream as shown.
As the TWC's efficiency degrades, its ability to store oxygen is reduced. This causes higher variation in post TWC exhaust stream oxygen content and results in increased sensor 2 signal activity, as shown above. When the running the monitor, the ECM compares sensor 1 and sensor 2 signals over a specific time to determine TWC efficiency. The ECM begins by calculating the signal length for both sensors.