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Catalyst Circuit Malfunction-MCC Deterioration
DTC
P0421
Catalyst Circuit Malfunction-MCC Deterioration (Bank 1)
DESCRIPTION
The ECU compares the waveform of the front HO2S with the waveform of the rear
HO2S to determine whether or not catalyst performance has deteriorated. Air-fuel ratio
feedback compensation keeps the waveform of the front HO2S repeatedly changing
back and forth from rich to lean.
If the catalyst is functioning normally, the waveform of the rear HO2S switches back and
forth between rich and lean much more slowly than the waveform of the front HO2S.
When both waveforms change at a similar rate, catalyst performance has deteriorated.
TROUBLESHOOTING GUIDE
DTC detection condition
Normal Operation
a. The signal from the rear HO2S differs from the front
HO2S. This is because the catalytic converter
purifies, the exhaust gas. When the catalytic
converter has deteriorated, the signal from the rear
becomes similar to that of the front.
b. ECU checks the outputs of both HO2S signals.
Normal Operation
a. Engine speed is 3,000 rpm or higher.
b. Closed throttle position switch: OFF
c. Closed loop operation
d. Monitoring Time: 140 sec.
Malfunction
a. The front and rear HO2S signals are similar.
TEST PROCEDURE
Diagnostic item
a. Catalytic converter
deteriorated
b. HO2S failed
c. ECU failed
Probable cause
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