engine operation step 50 preferably after the engine has warmed up to within the normal operating range step 52 the intervals at which the deterioration test are carried out depend upon the requirements for a particular engine and vehicle to begin the testing the engine is operated so that a limit cycled time domain signal is produced by the sensor step 54 this signal may be produced in two ways first the engine controller may force the engine operation to switch between rich and lean air fuel ratios at a predetermined frequency a forced oscillation mode in this way knowing where the peaks should occur in the fft signal will be easy or second the signal may also be produced when it is determined that the engine is in steady state operation where the frequency of the rich lean limit cycle can be determined then the location of the peaks in the fft signal can be determined next the time domain oxygen signal taken over a predetermined time interval undergoes a fft to create a frequency domain signal step 56 from this frequency domain signal the magnitudes of the first order harmonic p 1 the second order harmonic p 2 and the third order harmonic p 3 are calculated step 58 a ratio of the first to the second order harmonic p 1 p 2 is then calculated step 60 this ratio is compared to.