P0420 on a 2021 Ford F 150 Super Crew

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 Super Crew

Ranked from 941 NHTSA owner complaints filed for this vehicle.

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1
Long-term misfire damaging the catalyst substrate
High severity
Five complaints on this truck document confirmed misfire from multiple root causes (spark plugs, ignition coils, injectors, valve/timing damage); sustained unburned fuel from any of these overheats and melts the catalyst over time.
Reported in 5 NHTSA complaints
2
Oil contamination of the catalyst from excessive oil consumption
Medium severity
Four complaints describe the 5.0L Coyote V8 in this truck burning oil severely; oil passing into the exhaust can coat and poison the catalyst, dropping its efficiency below threshold.
Reported in 4 NHTSA complaints
3
Failing catalytic converter (widespread known defect on this platform)
High severity
One complaint documents Ford's own service program (24E04/24E08) acknowledging a widespread catalytic converter failure on this platform, including a PCM software update tied to the catalyst -- but this VIN was excluded from the covered program.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Worn/degraded catalytic converter (age and mileage)
Medium severity
Catalyst efficiency naturally declines with age/mileage until it can no longer hold the downstream O2 signal in spec.
Common cause
Parts:catalytic converter
5
Failing downstream (post-catalyst) O2 sensor
Medium severity
A lazy or biased downstream sensor can mimic reduced catalyst efficiency even with a healthy catalyst; compare upstream/downstream switching rates.
Common cause
Parts:oxygen sensor
6
Exhaust leak ahead of the sensor/catalyst
Low severity
Any leak upstream dilutes the exhaust sample the O2 sensors see, which can present as a false catalyst-efficiency fault.
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