Method and system for detecting fault of rear oxygen sensor

Abstract

A method for detecting a fault of a rear oxygen sensor installed on an exhaust pipe of a vehicle may include determining whether the rear oxygen sensor satisfies a diagnosis entry condition, comparing an output voltage of the rear oxygen sensor to a preset fault determination voltage range so as to determine whether the output voltage of the rear oxygen sensor is included in the fault determination voltage range, changing the duty of a heater of the rear oxygen sensor when the output voltage of the rear oxygen sensor is not included in the fault determination voltage range, determining whether the output voltage of the rear oxygen sensor, controlled by changing the heater duty, is included in the fault determination voltage range, and confirming that the rear oxygen sensor has a fault, when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.

What is claimed is: 
     
         1 . A method for detecting a fault of a rear oxygen sensor which is installed at the rear of a catalytic converter on an exhaust pipe of a vehicle and measures the concentration of oxygen included in exhaust gas passing through the catalytic converter, the method comprising:
 determining whether the rear oxygen sensor satisfies a diagnosis entry condition;   comparing an output voltage of the rear oxygen sensor to a preset fault determination voltage range so as to determine whether the output voltage of the rear oxygen sensor is included in the fault determination voltage range;   changing the duty of a heater of the rear oxygen sensor when the output voltage of the rear oxygen sensor is included in the fault determination voltage range;   determining whether the output voltage of the rear oxygen sensor, controlled by changing the heater duty, is included in the fault determination voltage range; and   confirming that the rear oxygen sensor has a fault, when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.   
     
     
         2 . The method of  claim 1 , wherein in the determining whether the rear oxygen sensor satisfies the diagnosis entry condition,
 the diagnosis entry condition is satisfied when the temperature of the rear oxygen sensor exceeds a preset temperature.   
     
     
         3 . The method of  claim 1 , wherein in the determining whether the rear oxygen sensor satisfies the diagnosis entry condition,
 the diagnosis entry condition is satisfied when an engine is completely warmed up.   
     
     
         4 . The method of  claim 1 , wherein in the determining whether the rear oxygen sensor satisfies the diagnosis entry condition,
 the diagnosis entry condition is satisfied when control for an air/fuel ratio of an engine is performed.   
     
     
         5 . The method of  claim 1 , wherein in the determining whether the rear oxygen sensor satisfies the diagnosis entry condition,
 the diagnosis entry condition is satisfied when the vehicle is not coasting.   
     
     
         6 . The method of  claim 1 , wherein in the comparing the output voltage of the rear oxygen sensor to the preset fault determination voltage range so as to determine whether the output voltage of the rear oxygen sensor is included in the fault determination voltage range,
 the procedure returns to the determining whether the rear oxygen sensor satisfies the diagnosis entry condition when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.   
     
     
         7 . The method of  claim 1 , wherein in the changing the duty of the heater of the rear oxygen sensor,
 the duty of the heater is changed to minimum from maximum when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.   
     
     
         8 . The method of  claim 1 , wherein the changing the duty of the heater of the rear oxygen sensor is performed while the rear oxygen sensor maintains an activation temperature. 
     
     
         9 . The method of  claim 8 , wherein in the changing the duty of the heater of the rear oxygen sensor,
 the duty of the heater is changed for 4 seconds to 6 seconds when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.   
     
     
         10 . The method of  claim 1 , further comprising, between the steps of comparing the output voltage of the rear oxygen and the changing the duty of the heater of the rear oxygen sensor when the output voltage of the rear oxygen sensor is included in the fault determination voltage range:
 determining that the rear oxygen sensor has a fault, when the output voltage of the rear oxygen sensor is included in the fault determination voltage range, and increasing a fault diagnosis count; and   comparing the fault diagnosis count to a preset heater duty change count so as to determine whether the fault diagnosis count is larger than the heater duty change count.   
     
     
         11 . The method of  claim 10 , wherein in the determining whether the output voltage of the rear oxygen sensor, controlled by changing the heater duty, is included in the fault determination voltage range,
 when the output voltage of the rear oxygen sensor is not included in the fault determination voltage range,   the fault diagnosis count is reset, and the procedure returns to the determining whether the rear oxygen sensor satisfies the diagnosis entry condition.   
     
     
         12 . The method of  claim 1 , wherein in the comparing the output voltage of the rear oxygen sensor to the preset fault determination voltage range so as to determine whether the output voltage of the rear oxygen sensor is included in the fault determination voltage range and the determining whether the output voltage of the rear oxygen sensor, controlled by changing the heater duty, is included in the fault determination voltage range,
 the fault determination voltage ranges are set to the same range.   
     
     
         13 . A system for detecting a fault of a rear oxygen sensor, comprising:
 a voltage comparator configured to determine whether an output voltage of a rear oxygen sensor is included in a preset fault determination voltage range;   a counter configured to increase a fault diagnosis count, when the voltage comparator determines that the output voltage of the rear oxygen sensor is included in the fault determination voltage range, and determine whether the fault diagnosis count reaches a previously stored heater duty change count;   a heater power controller configured to control power applied to a heater installed at the rear oxygen sensor such that the duty of the power is decreased, when the fault diagnosis count reaches the heater duty change count; and   a controller configured to (i) determine whether a condition for diagnosing a fault of the rear oxygen sensor is satisfied, based on information inputted from an electronic control unit (ECU) of a vehicle, (ii) control the counter to increase the fault diagnosis count when the voltage comparator determines that the output voltage of the rear oxygen sensor is included in the fault determination voltage range, and (iii) operate the heater power controller to decrease the duty of the heater, when the fault diagnosis count stored in the counter reaches the preset heater duty change count.   
     
     
         14 . The system of  claim 13 , wherein the heater power controller decreases the duty of the power applied to the heater to minimum for a preset time. 
     
     
         15 . The system of  claim 13 , wherein the heater power controller decreases the duty of the power applied to the heater for a time in which the rear oxygen sensor maintains an activation temperature.