NOx sensor diagnostic device and exhaust gas purifying system using the device

An NOx sensor diagnostic device for an internal combustion engine, comprising:

• an NOx sensor disposed in an exhaust passage of the engine;
• an output value obtaining means for obtaining an output value of the NOx sensor;
• a spray characteristics changing means for changing spray characteristics for a specific cylinder of a plurality of cylinders of the engine;
• and
• a diagnostic means for diagnosing abnormality of the NOx sensor, based on the output value of the NOx sensor that is changed as a result of the change of the spray characteristics for the specific cylinder by the spray characteristics changing means, wherein:
• the diagnostic means diagnoses the abnormality of the NOx sensor based also on a response time for the change of the output value of the NOx sensor in response to the change of the spray characteristics for the specific cylinder;
• and
• the response time is one of:
• a decrease response time from a start of the change of the spray characteristics for the specific cylinder to a start of the change of the output value of the NOx sensor;
• and
• a recovery response time from an end of the change of the spray characteristics for the specific cylinder to an end of the change of the output value of the NOx sensor.