Offset and slow response diagnostic methods for NOx sensors in vehicle exhaust treatment applications

1. A method of diagnosing vehicle NOx sensor faults, comprising the steps of:

(a) determining that the fuel flow to the vehicle engine is interrupted using an engine control unit (ECU) to monitor a fuel flow sensor;

(b) monitoring a NOx sensor at the ECU to determine if the output from the NOx sensor remains above an upper threshold for more than a predetermined amount of time; and

(c) if so, then deciding at the ECU that a high-offset error exists for the NOx sensor;

(d) otherwise, deciding at the ECU that a low-offset error exists if the output from the NOx sensor falls below a zero or negative threshold for more than a predetermined amount of time.

2. The method of claim 1 , wherein step (b) further comprises using a timer included with an engine control unit (ECU) to determine if the output from the NOx sensor remains above an upper threshold.

3. The method of claim 1 , wherein the upper threshold or negative threshold is stored in memory on an engine control unit (ECU).

4. The method of claim 1 , further comprising the step of triggering a flag for alerting a vehicle occupant of the low-offset error or the high-offset error.

5. The method of claim 1 , further comprising the step of determining if the duration of steps (a)-(d) exceeds a maximum allotted time.

6. A method of diagnosing vehicle NOx sensor faults, comprising the steps of:

(a) determining the rate of fuel flow to the vehicle engine using an engine control unit (ECU) to monitor a fuel flow sensor;

(b) receiving the output of the NOx sensor at the ECU;

(c) calculating the fourth moment of the rate of fuel flow to the vehicle engine and the fourth moment of the rate of output change for the NOx sensor over a plurality of data points using the ECU;

(d) calculating at the ECU a ratio between the fourth moment of the rate of fuel flow and the fourth moment of the rate of output change for the NOx sensor;

(e) comparing at the ECU the calculated ratio to a threshold; and

(f) if the calculated ratio is greater than the threshold, then determining at the ECU that an error condition exists.

7. A method of diagnosing vehicle NOx sensor faults, comprising the steps of:

(a) passing the output signal from the NOx sensor through a magnification filter;

(b) detecting the minimum values of output from the magnification filter using a microprocessor;

(c) determining whether the minimum values are negative using the microprocessor; and

(d) if the minimum values are not negative, then determining that a fault condition exists with the NOx sensor.

8. The method of claim 7 , wherein the minimum values are detected using an envelope filter.

9. The method of claim 7 , further comprising the steps of: calculating the mean of the minimum values and determining if the mean of the minimum value is below a predetermined threshold.

10. A method of diagnosing vehicle NOx sensor faults, comprising the steps of:

(a) detecting that fuel has stopped flowing to the vehicle engine;

(b) measuring the output from the NOx sensor over a predetermined amount of time;

(c) calculating the slope of the measured output of the NOx sensor based on time and NOx sensor output;

(d) determining if the calculated slope is less than a predetermined slope value; and

(e) if so, then deciding that a sensor is not performing properly, wherein steps (c)-(e) are carried out using an engine control unit (ECU) or a microprocessor.

11. The method of claim 10 , wherein the slope of the output is calculated using the mean of the measured output from the NOx sensor.

12. The method of claim 10 , wherein the slope of the output is calculated using the minimum value or the maximum value of the measured output from the NOx sensor.