FAILURE DIAGNOSIS DEVICE OF EMISSION CONTROL SYSTEM

In a failure diagnosis device of an emission control system that utilizes an electrode-based PM sensor (54) provided downstream of a particulate filter (51) in an exhaust conduit to diagnose a failure of the particulate filter, an object of the invention is to suppress reduction of accuracy of diagnosis of a failure due to in-cylinder rich control. The failure diagnosis device of the emission control system starts application of a predetermined voltage to electrodes of the electrode-based PM sensor (54) after a sensor recovery process that removes PM depositing between the electrodes of the PM sensor, and diagnoses a failure of the particulate filter based on an output of the PM sensor measured after elapse of a predetermined time period (ts) since the start of application of the predetermined voltage. The failure diagnosis device performs the sensor recovery process during in-cylinder rich control or triggered by termination of the in-cylinder rich control and subsequently performs a measurement process after termination of the in-cylinder rich control and the sensor recovery process.