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

WARNING: This page is about a different car, the 2011 Mercury Milan, 2011 Lincoln MKZ, 2011 Ford Fusion, 2010 Mercury Milan, and 2010 Ford Fusion. However, it is still accessible from the selected car via links, so may be relevant.

P0420 - CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 1)

Description:	Indicates the catalyst system efficiency is below the acceptable threshold.
Possible Causes:	Use of leaded fuel Damaged heated oxygen sensor (HO2S) Out of range cylinder head temperature (CHT) sensor High fuel pressure Damaged exhaust manifold Damaged catalytic converter Oil contamination Cylinder misfiring Damaged exhaust system pipe Damaged muffler/tailpipe assembly
Diagnostic Aids:	Under normal closed loop fuel conditions, high efficiency catalysts have a high level of oxygen storage that makes the switching frequency of the rear HO2S very slow and reduces the amplitude of the switches. As catalyst efficiency deteriorates, its ability to store oxygen declines and the rear HO2S signal begins to switch more rapidly with increasing amplitude. The powertrain control module (PCM) compares the signal line length of the rear HO2S to an expected signal line length of a rear HO2S with a deteriorated catalytic converter. Once beyond an acceptable limit the DTC is set.
Application	Key On Engine Off	Key On Engine Running	Continuous Memory
All	GO to PINPOINT TEST HF .