Patent ID: 8266953

Claim:
A method for detecting an abnormality of a nitrogen oxide purifying catalyst using an NOx sensor for outputting an output signal in accordance with nitrogen oxide concentration, the nitrogen oxide purifying catalyst being disposed in a discharge passage of an internal combustion engine discharging an exhaust gas and reducing nitrogen oxide contained in the exhaust gas using a reducing agent during activation of the internal combustion engine, the method comprising: a detection integrated value calculating step which comprises calculating a detection integrated value by integrating corresponding values corresponding to the NOx sensor output signal obtained until a first predetermined detection period has elapsed after the NOx sensor begins to generate an output signal during activation of the internal combustion engine; and an abnormality detecting step which comprises subtracting a variation integrated value from the detection integrated value calculated in the detection integrated value calculating step and detecting an abnormality of the nitrogen oxide purifying catalyst on the basis of the subtracted value, where the variation integrated value is calculated in advance to represent an amount of variation in the output signal in an unstable period during which the output signal changes independent of nitrogen oxide concentration after the NOx sensor begins to generate an output signal, and the variation integrated value is calculated by integrating corresponding values obtained until a predetermined second detection period set to be the same as the first detection period has elapsed after the NOx sensor begins to generate an output signal when driven in a gas atmosphere having a constant nitrogen oxide concentration.