P0420 Ford - most common reason for code PO420 to appear

Question

most common cause for the check engine light code PO420 to appear

Answer 1

P0420 Catalytic below threshold Bank 1

When it comes to a Ford, the common cause is Motor Oil. Ford in 2007 sued API , which they blame  the use of zinc in motor oil  that cause their catalytic convertor to fail early. Zinc is now removed in motor oil and it is up to the consumer to have the proper motor oil type installed in the engine. Another is a prolong misfire this is devaestating to any engine. But when it comes to a catalytic code, the motor oil is mostly the blame.  not in maintance but in type.