Circuit Description

Engine Control Module (ECM) compares waveform pattern of heated Air/Fuel (A/F) sensor located on exhaust pipe in front of catalytic converter with waveform pattern of heated oxygen sensor located on exhaust pipe behind catalytic converter to determine if catalytic converter performance has deteriorated. If catalytic converter is functioning properly, waveform pattern for heated oxygen sensor on exhaust pipe should switch back and forth from rich to lean more slowly than A/F sensor. If both waveform patterns switch at a similar rate, catalytic converter performance has deteriorated.