Circuit Description
The ECM uses Heated Oxygen (HO2) sensor mounted in front of and behind the Three-Way Catalytic Converter (TWC) to monitor its efficiency. The front sensor (sensor 1) sends pre-catalyst air-fuel information to the ECM. The rear sensor (sensor 2) sends post-catalyst information to the ECM. The ECM compares these two signals to determine the efficiency of the TWC and the TWC's ability to store oxygen. During normal operation, the TWC stores and releases oxygen as needed. This capacity to store oxygen results in a low variation in the post-TWC exhaust stream as shown.
If the TWC is functioning normally, the waveform of the rear HO2 sensor (sensor 2) slowly alternates between RICH and LEAN. If the TWC has deteriorated, the waveform alternates frequently between RICH and LEAN. As the catalyst efficiency degrades, its ability to store oxygen decreases and the TWC output becomes more variable.
When the TWC monitor is activated, the ECM begins to measure the signal lengths of sensor 1 and sensor 2 signals. The ECM calculates the ratio of those signal lengths (catalyst deterioration level). If the catalyst deterioration level exceeds the threshold, the ECM interprets this as a malfunction of the TWC. The ECM illuminates the MIL (2 trip detection logic) and sets the DTC.