P0420 on a 2021 Ford F 150 Super Cab Diesel

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 Cab Diesel

Ranked from 941 NHTSA owner complaints filed for this vehicle.

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1
Oil or coolant contamination of the catalyst from chronic consumption/internal leaks
High severity
This truck's owners report chronic oil consumption (down to no oil on the dipstick) and an unexplained, unresolved loss of engine coolant with a burning-coolant smell -- both can coat and poison a catalyst over time.
Reported in 4 NHTSA complaints
2
Repeated cylinder misfires overheating/damaging the catalyst substrate
Medium severity
This truck has documented misfires (spark plugs, coils, injectors replaced) and a low-compression cylinder from valve/piston damage; unburned fuel from ongoing misfires can overheat and melt a catalyst.
Reported in 4 NHTSA complaints
3
Worn/failed catalytic converter (documented widespread failure on this platform)
High severity
One owner's converter (part 24E08) was diagnosed as failed under a known widespread-failure customer-service program, with Ford also flagging a related PCM software update -- direct evidence of catalyst failure on this vehicle.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Failing downstream (post-cat) O2 sensor
Medium severity
A degraded rear sensor can mimic a dead catalyst on the efficiency monitor even with a healthy converter.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leaking joint or gasket before the rear sensor lets in fresh air that skews the efficiency comparison.
Common cause
6
Leaking fuel injector causing a rich condition that overloads the catalyst
Medium severity
Excess unburned fuel from a dribbling injector overwhelms the catalyst's oxidizing capacity, showing up as an efficiency fault.
Common cause
Parts:fuel injector

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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