P0421
DESCRIPTIONThe ECM compares the wave form of the oxygen sensor located before the catalyst with the wave form of the oxygen sensor located after the catalyst to determine whether or not catalyst performance has deteriorated.
Air-fuel ratio feedback compensation keeps the wave form of the oxygen sensor before the catalyst repeatedly changing back and forth from rich to lean.
If the catalyst is functioning normally, the wave form of the oxygen sensor after the catalyst switches back and forth between rich and lean much more slowly than the waveform of the oxygen sensor before the catalyst.
But when both wave forms change at a similar rate, it indicates that catalyst performance has deteriorated.
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