Patent ID: 8490386

Claim:
A method for detecting abnormality in a reducing agent replenished into a reducing agent tank in an exhaust emission control device for reduction and removal of NO x through addition of the reducing agent from said tank to a selective reduction catalyst incorporated in an exhaust pipe, comprising: detecting replenishment of the reducing agent to said tank; waiting elapse of a removal-rate stabilization period as a period from the detected replenishment to stabilization of the NO x removal rate; and determining that a dilute reducing agent or/and a different material are replenished into the tank if an abnormal lowering of a measured NO x removal rate is detected during a determination period after the removal-rate stabilization period, wherein, when the abnormal lowering of the measured NO x removal rate cannot be detected, the actually measured NO x removal rate is recorded as a normal NO x removal rate to the reference NO x removal rate during a normal operation period after the determination period.