DIAGNOSTIC DEVICE FOR NOx SENSOR

Abstract

The ECU performs an on-board diagnosis for a NOx sensor. The diagnosis is carried out when an exhaust system of an engine is fully cooled after shut-down of the engine. The diagnosis is carried out after a temperature in the exhaust passage is lowered to an ambient temperature. In the diagnosis proceeding, the ECU activates a heater attached on the NOx sensor. The ECU detects a heating performance of the heater and determines whether the heating performance is sufficient or not. The NOx sensor is in a stable condition after the engine system is fully cooled. Therefore, it is possible to perform the diagnosis while eliminating or suppressing influences of operating conditions of the engine system and a residual heat in the exhaust system.

1 . A diagnostic device for a NOx sensor including a NOx sensitive element sensitive to a concentration of NOx components in exhaust gas from an internal combustion engine and a heater for heating the NOx sensitive element to an active temperature, comprising:
 means for detecting a temperature of the NOx sensor;   means for detecting an ambient temperature; and   means for diagnosing a heating performance of the heater by supplying power to the heater when the internal combustion engine is shut-down, and a difference between the temperature of the NOx sensor and the ambient temperature is lower than or equal to a predetermined threshold temperature.   
   
   
       2 . The diagnostic device for the NOx sensor claimed in  claim 1 , wherein
 the diagnosing means diagnoses the heating performance of the heater based on a reaching temperature of the heater for a predetermined time of power supply.   
   
   
       3 . The diagnostic device for the NOx sensor claimed in  claim 2 , wherein
 the diagnosing means includes means for correcting the reaching temperature in accordance with the ambient temperature.   
   
   
       4 . The diagnostic device for the NOx sensor claimed in  claim 1 , wherein
 the diagnosing means diagnoses the heating performance of the heater based on a reaching time period of power supply until the temperature of the heater reaches to a predetermined threshold temperature.   
   
   
       5 . The diagnostic device for the NOx sensor claimed in  claim 4 , wherein
 the diagnosing means includes means for correcting the reaching time period in accordance with the ambient temperature.   
   
   
       6 . The diagnostic device for the NOx sensor claimed in  claim 1 , wherein
 the NOx sensor temperature detecting means detects the temperature based on a period of time from shut-down of the internal combustion engine.   
   
   
       7 . The diagnostic device for the NOx sensor claimed in  claim 1 , wherein
 the NOx sensor temperature detecting means includes a sensor directly or indirectly detects a temperature in an exhaust passage through which an exhaust gas flows, and   the diagnosing means further includes means for determining whether the diagnosis for the heating performance of the heater is permitted or not based on a difference between the temperature in the exhaust passage detected by the sensor and the ambient temperature.   
   
   
       8 . The diagnostic device for the NOx sensor claimed in  claim 1 , wherein
 the diagnosing means permits the diagnosis for the heating performance of the heater when the temperature of the NOx sensor is almost the same as the ambient temperature.   
   
   
       9 . The diagnostic device for the NOx sensor claimed in  claim 1 , wherein
 the diagnosing means terminates the diagnosis for the heating performance of the heater when the ambient temperature is lower than or equal to a predetermined threshold temperature.   
   
   
       10 . The diagnostic device for the NOx sensor claimed in  claim 1 , wherein
 the diagnosing means terminates the diagnosis for the heating performance of the heater when a charge level of a battery is lower than or equal to a predetermined threshold level.