| abstract |
In an exhaust purification system in which urea water is added to a selective reduction type NOx catalyst formed in an exhaust passage of an internal combustion engine by a urea addition device, a sensor for detecting ammonia concentration downstream of the NOx catalyst is formed. Then, a predetermined amount of urea water is added during the fuel cut, and at least one abnormality of the urea addition device and urea water is detected based on the sensor output at this time. By adding urea water in the fuel cut, reaction consumption of urea with NOx is prevented, and the output of the sensor corresponding to the amount of urea is obtained. By comparing this output state with a normal output state, it can be detected whether urea water of an appropriate amount and quality has been added. |