6-88 ENGINE GENERAL INFORMATION AND DIAGNOSIS
DTC P0420 Catalyst System Efficiency below Threshold
CIRCUIT DESCRIPTION
ECM monitors oxygen concentration in the exhaust gas which has passed the three way catalytic converter by
HO2S-2.
When the catalyst is functioning properly, the variation cycle of HO2S-2 output voltage (oxygen concentration) is
slower than that of HO2S-1 output voltage because of the amount of oxygen in the exhaust gas which has been
stored in the catalyst.
REFERENCE
1. Heated oxygen sensor-1 3. Three way catalytic converter 5. To the sensor [B] : Case of TYPE B is shown
(See NOTE)
2. Warm up three way catalytic
converter
4. Heated oxygen sensor-2 [A] : Case of TYPE A is shown
(See NOTE)
NOTE:
For TYPE A and TYPE B, refer to the NOTE in “ECM Terminal Voltage Values Table” for applicable
model.
1. Engine running at closed loop
condition
2. Fuel cut 3. Idle after fuel cut [A] : Oscilloscope waveforms
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