Monitor Description

The ECM uses the heated oxygen sensors mounted before and after the Three-Way Catalyst (TWC) to monitor its efficiency. The front sensor sends pre-catalyst air-fuel information to the ECM. The rear sensor sends post-catalyst information to the ECM. The ECM compares these 2 signals to judge the efficiency of the catalyst and the catalyst's ability to store oxygen. During normal operation, the TWC stores and releases oxygen as needed. The capacity to store oxygen results in a low variation in the post-TWC exhaust stream as shown below.

If the catalyst is functioning normally, the waveform of the heated oxygen sensor (sensor 2) slowly switches between rich and lean. If the catalyst is deteriorated, the waveform will alternate frequently between rich and lean.

As the catalyst efficiency degrades, its ability to store oxygen is reduced and the catalyst output becomes more variable.

When running the catalyst monitor, the ECM begins to measure the signal length of the heated oxygen sensor (sensor 1) and heated oxygen sensor (sensor 2). The ECM calculates the rate of the signal length of the heated oxygen sensor (sensor 1) and heated oxygen sensor (sensor 2) (catalyst deterioration level). If the catalyst deterioration level exceeds the threshold, the ECM interprets this as a catalyst malfunction. The ECM illuminates the MIL (2 trip detection logic) and sets a DTC.

The monitor runs after:

• The engine is warmed up (Engine Coolant Temperature (ECT) is 75°C (167°F) or more).
• The vehicle is driven at approximately 60 to 100 km/h (37 to 63 mph) for 15 minutes.

Fig 1: Identifying Waveforms Of Normal Catalyst & Deteriorated Catalyst

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

Fig 2: Heated Oxygen Sensor Signal Length

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002