P0420 on a 2021 Ford F 150 Regular Cab

P0420 means the downstream oxygen sensor pattern suggests the bank 1 catalytic converter is not storing oxygen properly.

Not your vehicle? See general P0420 info.

Most common causes on the 2021 Ford F 150 Regular Cab

Ranked from 941 NHTSA owner complaints filed for this vehicle.

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1
Excessive oil consumption contaminating the catalytic converter (5.0L Coyote V8, TSB 19-2365)
Medium severity
Owners document the engine running critically low on oil with no warning; oil pulled into the combustion chamber and burned is a well-known contributor to premature catalyst fouling.
Reported in 4 NHTSA complaints
Parts:catalytic converter
2
Prolonged engine misfires (cracked spark plugs) overheating/damaging the catalyst
High severity
Unburned fuel from cracked-spark-plug misfires ignites in the exhaust and can melt the catalyst substrate over time.
Reported in 3 NHTSA complaints
Parts:catalytic converter spark plugs
3
Known catalytic converter defect covered under Ford customer satisfaction program (24E04/24E08, PCM software update)
High severity
An owner outside the covered VIN range was told by Ford the converter and PCM software needed replacement/update for a widespread, documented catalyst failure on this platform.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Aged/worn catalytic converter (normal high-mileage wear)
Low severity
Catalyst efficiency degrades naturally with age and mileage; confirm with an upstream/downstream O2 switching comparison.
Common cause
Parts:catalytic converter
5
Failing downstream oxygen sensor
Low severity
A lazy or biased rear O2 sensor can mimic a catalyst-efficiency fault; compare its switching pattern to the upstream sensor.
Common cause
Parts:oxygen sensor
6
Exhaust leak ahead of the downstream sensor
Low severity
A leak upstream of the rear sensor dilutes its sample with outside air and can falsely trigger a catalyst code.
Common cause

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These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0420 on your car still needs shop confirmation before any repair.

Common symptoms

Check engine light
Usually no drivability issue
Failed emissions test
Sulfur smell in some cases
Reduced power if converter is clogged

What to check before replacing parts

Check for misfire and fuel trim codes first

Look for: Any active or recent misfire, or lean/rich trim codes, means raw fuel or an off mixture has likely already damaged or is currently overworking the converter — fix those before touching the catalyst.

Inspect for exhaust leaks ahead of the converter

Look for: A leak before the converter (cracked manifold, loose flange, blown gasket) lets in outside air that confuses the downstream O2 sensor into reading as if the catalyst isn't working, even when it is.

Compare upstream and downstream O2 sensor activity

Look for: A healthy converter makes the downstream sensor read flat and steady while the upstream sensor swings actively. If the downstream sensor switches nearly as fast as the upstream one, the converter isn't storing oxygen properly and is likely worn out.

Check converter temperature and performance

Look for: A converter outlet noticeably cooler than expected relative to its inlet under load suggests little catalytic reaction is happening inside — supports a genuinely failed converter rather than a false reading.

Hold off on replacing the converter until root causes are ruled out

Look for: If misfires, exhaust leaks, or fuel trim issues caused this code, a new converter will fail again quickly — confirm those are clean first.

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Matching symptoms

Check Engine Light