Patent ID: 7757478

Claim:
A method for monitoring operation of an exhaust gas treatment system using an exhaust gas sensor, the exhaust gas treatment system having an exhaust pipe configured to receive exhaust gases, an SCR catalyst coupled to the exhaust pipe, and a urea delivery system configured to deliver urea upstream the SCR catalyst, the exhaust gas sensor having an ammonia sensing electrode communicating with exhaust gases downstream of the SCR catalyst, the method comprising: generating a first output signal utilizing the ammonia sensing electrode, the first output signal being indicative of a first ammonia level; generating a second output signal to induce the urea delivery system to deliver a predetermined flow rate of urea upstream of the SCR catalyst, the second output signal being based on the first output signal; calculating a first ammonia level range based on an engine RPM value an exhaust as temperature value and an exhaust air/fuel ratio value; and generating a third output signal when the first ammonia level is outside the first ammonia level range, the third output signal indicating at least one of the urea delivery system having degraded operational performance, the SCR catalyst having degraded operational performance, and the exhaust gas sensor having degraded operational performance.