Monitor Description

The ECM uses sensors mounted in front of and behind the TWC (Three-Way Catalytic Converter) to monitor its efficiency.

The first sensor, the air fuel ratio sensor, sends pre-catalyst information to the ECM. The second sensor, the heated oxygen sensor, sends post-catalyst information to the ECM.

In order to detect any deterioration in the TWC, the ECM calculates the OSC (Oxygen Storage Capacity) of the TWC. This calculation is based on the voltage output of the heated oxygen sensor while performing active air-fuel ratio control.

The OSC value is an indication of the oxygen storage capacity of the TWC. When the vehicle is being driven with a warm engine, active air fuel ratio control is performed for approximately 15 to 20 seconds. When it is performed, the ECM deliberately sets the air fuel ratio to lean or rich levels. If a rich-lean cycle of the heated oxygen sensor is long, the OSC becomes greater. There is a direct correlation between the OSCs of the heated oxygen sensor and the TWC.

The ECM uses the OSC value to determine the state of the TWC. If any deterioration has occurred, it illuminates the MIL and stores the DTC.

This system determines the deterioration of the entire catalyst system (including the front and rear catalysts), by using the oxygen storage capacity value of the front catalyst, that is more sensitive than the rear catalyst, as the representative value. Therefore, be sure to replace the front and rear catalysts together when catalyst replacement is necessary.