methane hydrocarbon limit cycles is calculated by measuring the time period between the peaks in the methane limit cycle and the corresponding peaks in the non methane hydrocarbon limit cycle program control is then transferred to step 611 in step 611 the hydrocarbon and carbon monoxide conversion efficiencies for catalytic converter 34 are determined by using a look up table indexed as a function of engine rpm maf and the phase shift is calculated in step 609 this look up table is established a priori the resulting hydrocarbon conversion efficiency is compared with a threshold value in step 613 if during the execution of step 613 the hydrocarbon conversion efficiency is greater than the threshold value program control is transferred to step 619 where a malfunction indicator light mil is activated by engine control unit 10 program control is then transferred to end step 621 alternatively if the hydrocarbon conversion efficiency is less than or equal to the threshold value program control is transferred to step 615 where the carbon monoxide conversion efficiency determined in step 611 is compared with a threshold value if the carbon monoxide conversion efficiency is greater than a threshold value program control is transferred to step 619 where a malfunction indicator light mil is activated by engine control unit 10 program control is then transferred to end step 621 during the execution of step 615 if the carbon monoxide conversion efficiency determined in.