A method and system for monitoring a catalytic converter

A method and system are provided for monitoring exhaust gas conversion efficiency of a catalytic converter 30 during operation of an internal combustion engine 12 coupled to the catalytic converter. The system 10 includes an upstream exhaust gas sensor 32 interposed between the engine 12 and the catalytic converter 30 for generating a first signal based on the exhaust gas upstream of the converter 30 and a downstream exhaust gas sensor 34 interposed between the catalytic converter 30 and atmosphere for generating a second signal based on the exhaust gas downstream of the converter 30. A phase shift detector 46 is provided for estimating a phase shift between the first and second signals and a processor 26 is provided for aligning the first and second signals such that the resulting phase shift between the first and second signals is substantially an integer multiple of π radians and for determining a conversion efficiency of the catalytic converter 30 based on the phase aligned first and second signals.