JPH05240029

PURPOSE: To determine the performance loss of a catalytic converter precisely by multiplying a forward signal and a rearward signal of the catalytic converter with each other, averaging the products over a plurality of oscillations of the forward signal and determining the performance loss of the catalytic converter with the mean value. CONSTITUTION: A catalytic converter 10 with an oxygen probe 11.V mounted forward and an oxygen probe 11.H mounted rearward thereof, a value forming unit 12, an evaluation unit 13 and a map 14 are provided. The value forming unit 12 comprises a computation unit 15 and an operating condition detector 16. A forward signal λV of the catalytic converter from the forward probe 11.V and a rearward signal λH of the catalytic converter from the rearward probe 11.H are supplied to the computation unit 15. The forward signal λV and the rearward signal λH of the catalytic converter are multiplied with each other. The products are averaged over a plurality of oscillations of the forward signal. The mean value thus obtained is used as a value for evaluating the performance loss of the catalytic converter. When it is apparent that the performance loss of the catalytic converter 10 is beyond the allowable range, the computation unit 15 outputs a fault signal FS.