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Home >> Toyota >> 2001 >> Sienna XLE >> Repair and Diagnosis (Single Page) >> Engine Performance >> System >> Engine Control System Self-Diagnostics >> Diagnostic Tests >> DTC P0420: Catalyst System Efficiency Below Threshold >> Circuit Description
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Circuit Description

Engine Control Module (ECM) monitors waveform pattern of heated oxygen sensor (bank No. 1 sensor No. 2) located behind catalytic converter to determine if catalytic converter performance has deteriorated. If catalytic converter is functioning properly, the waveform pattern for heated oxygen sensor (bank No. 1 sensor No. 2) should switch back and forth from rich to lean more slowly than heated Air/Fuel (A/F) sensors. A/F sensors may also be referred to as A/F ratio sensors. If both waveform patterns switch at a similar rate, catalytic converter performance has deteriorated. DTC is set when A/F sensors and heated oxygen sensor (bank No. 1 sensor No. 2) have nearly the same waveform pattern after engine and catalytic converter are at normal operating temperature and vehicle is driven. Possible causes are:

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