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A system and method for controlling an internal combustion engine determine a catalyst gain based on an exhaust gas sensor positioned upstream relative to a catalyst and at least one exhaust gas sensor positioned downstream relative to at least a portion of the catalyst and use the gain to determine

A system and method for controlling an internal combustion engine determine a catalyst gain based on an exhaust gas sensor positioned upstream relative to a catalyst and at least one exhaust gas sensor positioned downstream relative to at least a portion of the catalyst and use the gain to determine the condition or performance of the catalyst. The gain may be determined by modeling the catalyst as an integrator with an unknown gain and estimating the gain using a polynomial approximation. The gain is compared to an expected value or threshold associated with current operating conditions, such as catalyst temperature and/or mass air flow.

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1. A method for controlling an internal combustion engine having an upstream exhaust gas sensor positioned upstream relative to a catalyst and at least one downstream exhaust gas sensor positioned downstream relative to at least a portion of the catalyst, the method comprising:modeling the catalyst

1. A method for controlling an internal combustion engine having an upstream exhaust gas sensor positioned upstream relative to a catalyst and at least one downstream exhaust gas sensor positioned downstream relative to at least a portion of the catalyst, the method comprising:modeling the catalyst as an integrator with an unknown gain; determining a catalyst gain based on the upstream and downstream exhaust gas sensors; and determining performance of the catalyst based on the catalyst gain. 2. The method of claim 1 wherein the step of determining the catalyst gain comprises:estimating the catalyst gain using a polynomial approximation. 3. The method of claim 1 wherein the step of determining performance of the catalyst comprises:determining a current value for at least one operating parameter indicative of current operating conditions; comparing the catalyst gain to a threshold associated with the operating parameter to determine performance of the catalyst. 4. The method of claim 3 wherein the at least one operating parameter comprises catalyst temperature.5. The method of claim 4 wherein the catalyst operating temperature is estimated.6. The method of claim 3 wherein the at least one operating parameter comprises catalyst temperature and mass air flow.7. The method of claim 1 further comprising controlling the engine based on the performance of the catalyst.8. A system for controlling an internal combustion engine, the system comprising:an upstream exhaust gas sensor for providing an indication of an air/fuel ratio upstream of a catalyst; a downstream exhaust gas sensor for providing an indication of an air/fuel ratio downstream of at least a portion of the catalyst; and a controller in communication with the upstream and the downstream exhaust gas sensors, the controller determining a catalyst gain based on the upstream and downstream exhaust gas sensors and comparing the gain to an expected value to determine condition of the catalyst. 9. The system of claim 8 wherein the upstream exhaust gas sensor is a universal exhaust gas oxygen sensor and wherein the downstream exhaust gas sensor is a heated exhaust gas oxygen sensor.10. The system of claim 8 further comprising:a catalyst monitoring exhaust gas sensor positioned downstream relative to the downstream exhaust gas sensor and in communication with the controller, wherein the controller determines a catalyst gain based on the upstream sensor and the catalyst monitoring sensor. 11. The system of claim 8 wherein the controller further determines current operating conditions and determines the condition of the catalyst by comparing the catalyst gain to an expected value for the current operating conditions.12. The system of claim 11 wherein determining current operating conditions includes determining catalyst temperature.13. The system of claim 11 wherein determining current operating conditions includes determining mass air flow.14. The system of claim 8 wherein the controller determines catalyst gain by modeling the catalyst as an integrator and estimating catalyst gain using a polynomial approximation.15. A computer readable storage medium having stored data representing instructions executable by a computer to control an internal combustion engine having a first exhaust gas oxygen sensor positioned upstream relative to a catalyst and a second exhaust gas oxygen sensor positioned downstream relative to the catalyst, the computer readable storage medium comprising:instructions for determining a catalyst gain based on the first and second exhaust gas oxygen sensors; and instructions for determining condition of the catalyst based on the catalyst gain. 16. The computer readable storage medium of claim 15 further comprising:instructions for determining a current value of an operating parameter; and instructions for comparing the catalyst gain to a threshold associated with the current value of the operating parameter to determine the condition of the catalyst. 17. The computer readable storage medium of claim 16 wherein the instructions for determining the current value of an operating parameter determine catalyst temperature.18. The computer readable storage medium of claim 16 wherein the instructions for determining the current value of an operating parameter determine mass air flow.19. The computer readable storage medium of claim 16 further comprising:instructions for modifying air/fuel ratio control of the engine based on the condition of the catalyst. 20. The computer readable storage medium of claim 16 wherein the instructions for determining the catalyst gain include instructions for estimating the catalyst gain using a polynomial approximation.