Patent ID: 7121080

Claim:
A computer readable storage medium having stored data representing instructions executable by a computer to monitor a catalytic device coupled in an exhaust of an internal combustion engine of a vehicle, said storage medium comprising: instruction for operating the engine lean; instructions for ending said lean operation when an amount of NOx stored in the catalyst approaches a maximum capacity for NOx storage; instructions for determining a lean operation time; instructions for operating the engine rich after said lean operation is ended; instructions for ending said rich operation when stored NOx is released based on a downstream air-fuel sensor; instructions for determining a rich operation time; instructions for correcting said rich operation time based on a level of richness during said rich operation and an exhaust gas flow rate during said rich operation; instructions for correcting said lean operation time based on a NOx concentration during said lean operation and an exhaust gas flow rate during said lean operation; instructions for determining a ratio between said corrected lean time and said corrected rich time; and instructions for determining degradation of the catalyst based on said ratio.