Circuit Description

WARNING: This page is about a different car, the 2002 Toyota Highlander. However, it is still accessible from the selected car via links, so may be relevant.

ECM compares waveform of Air/Fuel (A/F) sensor located on exhaust manifold above catalytic converter with waveform of heated oxygen sensor No. 2 located below catalytic converter to determine if catalytic converter performance has deteriorated. If both waveforms change at similar rate, catalyst performance has deteriorated. DTC P0420 applies to bank No. 1 and DTC P0430 applies to bank No. 2. Heated A/F sensors are located on exhaust manifold above catalytic converters. A/F sensor for cylinders No. 1 and 4 is referred to as bank No. 1 sensor No. 1. A/F sensor for cylinders No. 2 and 3 is referred to as bank No. 2 sensor No. 1. A/F sensors may also be referred to as A/F ratio sensors. Heated oxygen sensors are located on exhaust manifold below catalytic converters. Heated oxygen sensor for cylinders No. 1 and 4 is referred to as bank No. 1 sensor No. 2. Heated oxygen sensor for cylinders No. 2 and 3 is referred to as bank No. 2 sensor No. 2. In testing procedure, these heated oxygen sensors are referred to as heated oxygen sensor No. 2. DTC is set when A/F sensor and heated oxygen sensor No. 2 waveforms have same amplitude after vehicle is driven for 5 minutes at 20-50 MPH. Possible causes are:

Exhaust system leak.
Defective A/F sensor.
Defective heated oxygen sensor No. 2.
Defective catalytic converter.