Monitor Description

The ECM uses the diagnosis value to determine catalyst deterioration. If the diagnostic value exceeds the standard value, the ECM determines that there is catalyst degradation, illuminates the MIL, and stores the DTC.

Fig 1: Air Fuel Ratio Sensor Output Waveform

Courtesy of SUBARU OF AMERICA, INC.

The diagnosis value is calculated based on the catalyst monitor value (CM) which is determined from the amount of change in the air/fuel ratio sensor lambda value and oxygen sensor value, as well as the reaction rate determined during oxygen sensor reaction time.

Fig 2: Catalyst Monitor Value Calculation Formula

Courtesy of SUBARU OF AMERICA, INC.

*: Diagnosis value = CM x(1 + Reaction rate)