DTC P0420: Catalyst System Efficiency Below Threshold (Bank 1)

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P0420 - CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 1)

Description:	Indicates the bank 1 catalyst system efficiency is below the acceptable threshold.
Possible Causes:	Use of leaded fuel Damaged heated oxygen sensor (HO2S) Out of range engine coolant temperature (ECT) sensor High fuel pressure Damaged exhaust manifold Damaged catalytic converter Oil contamination Cylinder misfiring Downstream HO2S wires improperly connected Damaged exhaust system pipe Damaged muffler/tailpipe assembly Retarded spark timing Leaking fuel injector
Diagnostic Aids:	Compare HO2S upstream HO2S11 and downstream HO2S12 switch rate and amplitude. Under normal closed loop fuel conditions, high efficiency catalysts have oxygen storage which makes the switching frequency of the downstream HO2S very slow and reduces the amplitude of those switches as compared to the upstream HO2S. As catalyst efficiency deteriorates, its ability to store oxygen declines and the downstream HO2S signal begins to switch more rapidly with increase amplitude, approaching the switching rate and amplitude of the upstream HO2S. Once beyond an acceptable limit the DTC is set.
Application	Key On Engine Off	Key On Engine Running	Continuous Memory
All	-	-	GO to HF: Catalyst Efficiency Monitor and Exhaust Systems STEP HF1.