In a PM filter arranged in an exhaust passage of an internal combustion engine, failure such as breakage, erosion or the like may occur due to heat deterioration and time degradation. As a technique of detecting a failure of such a PM filter, there has been known one in which a failure is detected based on a difference in the exhaust gas pressure between the upstream side and the downstream side of the PM filter. If the PM filter is in failure, the flow rate of an exhaust gas passing through the PM filter will increase, so that the difference in the exhaust gas pressure between the upstream side and the downstream side of the PM filter will become smaller in comparison with the case where the PM filer is normal. Accordingly, the failure of the PM filter can be detected based on the pressure difference.
However, if the extent of the failure of the PM filter is not relatively so large, there will not be a clear difference in value of such a pressure difference in comparison with the case where the PM filter is normal. Thus, in recent years, there has been made a proposal that, in order to detect the failure of a PM filter, a PM sensor is used which serves to detect an amount of particulate matter in an exhaust gas for the failure detection.
For example, in Patent Document 1, there is disclosed a technique in which the failure of a PM filter is determined based on the amount of particulate matter detected by a PM sensor which is arranged at the downstream side of the PM filter. In addition, in this Patent Document, there is described that the failure determination of the PM filter is prohibited when the amount of accumulation or deposition of particulate matter in the PM filter is equal to or less than a reference amount of deposition.
In addition, in Patent Document 2, there is disclosed a technique in which the failure of a PM trapper is determined based on a ratio between a detected value of an input PM sensor arranged at the upstream side of the PM trapper, and a detected value of an output PM sensor arranged at the downstream side of the PM trapper.