Patent ID: 7788902

Claim:
A method for detecting abnormality in an exhaust emission control device for reduction and purification of NO x through addition of a reducing agent to selective reduction catalyst incorporated in an exhaust pipe, comprising monitoring a temperature of the catalyst during an operation period to record cumulative time for each of temperature zones, determining a deterioration coefficient of NO x reduction performance for each of the temperature zone on the basis of the recorded cumulative time for each of the temperature zones, multiplying a standard NO x reduction ratio predetermined for each of the temperature zones by said determined deterioration coefficient for each of the temperature zones to update the standard NO x reduction ratio, using the updated standard NO x reduction ratio in a next operation to compare an actually measured NO x reduction ratio with the standard NO x reduction ratio on a measured temperature zone, and determining presence of abnormality when deviation of the actually measured NO x reduction ratio from the standard NO x reduction ratio does not fall within a required range.