Monitor Description

The ECM uses 2 sensors mounted before and after the three-way catalytic converter (TWC) to monitor its efficiency. The air-fuel ratio (A/F) sensor (sensor 1) sends pre-catalyst information to the ECM. The heated oxygen (O2) sensor (sensor 2) sends post-catalyst information to the ECM.

In order to detect deterioration in the catalyst, the ECM calculates Oxygen Storage Capacity (OSC) in the catalyst based on voltage output of sensor 2 while performing "active air-fuel ratio control" instead of the conventional detecting method which uses the locus ratio.

The OSC is an indication value of the catalyst oxygen storage capacity and is used for representing how much the catalyst can store oxygen. When the vehicle is being driven with a warm engine, the active air-fuel ratio control is performed for approximately 15 to 20 seconds. When it is performed, the air-fuel ratio is forcibly regulated to be LEAN or RICH by the ECM, and if a RICH and LEAN cycle of sensor 2 is long, the OSC will become greater. The greater OSC and capability of the catalyst are mutually related. The ECM judges if the catalyst has deteriorated based on the calculated OSC value. The ECM will illuminate the MIL and a DTC will be set.

HINT:

• Sensor 1 refers to the sensor mounted before the TWC and is located near the engine assembly.
• Sensor 2 refers to the sensor mounted after the TWC and is located far from the engine assembly.