Here, from Ford TSB 01-9-7, are the possible causes of P0420/P0430.
Quote:
Malfunctioning ECT/CHT
•High fuel pressure
•Damaged exhaust manifold
•Cylinder misfiring
•HO2S wiring concerns (shorted or chafed,
bent pins, etc.)
•Damaged exhaust system pipe
•Damaged muffler/tailpipe assembly
•Retarded spark timing
Damaged Catalytic Converter:
•Use of leaded fuel
•Oil contamination/consumption
•Silicone contamination (sealants/cleaners)
Also from the same TSB:
Quote:
Do not replace downstream HO2S sensors for DTCs P0420 and/or
P0430: When diagnosing a vehicle with a
customer concern of MIL On and DTCs
P0420/P0430 in continuous memory, do not
replace the downstream HO2S sensors
(HO2S12/HO2S22). Damaged or malfunctioning
downstream HO2S sensors will not cause these
DTCs to be set. Always verify the vehicle
concern, then perform the pinpoint diagnostics
in the appropriate PC/ED Service Manual.
Quote:
Malfunctioning ECT/CHT
•High fuel pressure
•Damaged exhaust manifold
•Cylinder misfiring
•HO2S wiring concerns (shorted or chafed,
bent pins, etc.)
•Damaged exhaust system pipe
•Damaged muffler/tailpipe assembly
•Retarded spark timing
Damaged Catalytic Converter:
•Use of leaded fuel
•Oil contamination/consumption
•Silicone contamination (sealants/cleaners)
Also from the same TSB:
Quote:
Do not replace downstream HO2S sensors for DTCs P0420 and/or
P0430: When diagnosing a vehicle with a
customer concern of MIL On and DTCs
P0420/P0430 in continuous memory, do not
replace the downstream HO2S sensors
(HO2S12/HO2S22). Damaged or malfunctioning
downstream HO2S sensors will not cause these
DTCs to be set. Always verify the vehicle
concern, then perform the pinpoint diagnostics
in the appropriate PC/ED Service Manual.