P0420 on a 2018 Ford F 250 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 2018 Ford F 250 Regular Cab

Ranked from 382 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Long-term fuel-starvation misfires overheating/damaging the catalyst substrate
High severity
The same notorious HPFP/LPFP failures that stall this truck typically cause rough-running, fuel-starved misfire episodes beforehand; repeated unburned-fuel events like these are a recognized way this generation's catalysts get cooked.
Reported in 4 NHTSA complaints
2
Worn or failed catalytic converter
High severity
A dealer directly diagnosed a defective catalytic converter after the truck displayed an 'exhaust failure, service now' message and shut off.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A degraded rear O2 sensor can no longer confirm the catalyst is working even when it still is.
Common cause
Parts:oxygen sensor
4
Exhaust leak upstream of the sensor
Medium severity
A leak ahead of the downstream sensor introduces outside air that skews the catalyst efficiency reading.
Common cause
5
Oil or coolant contamination of the catalyst
Medium severity
Internal oil or coolant leaks that reach the exhaust coat and poison the catalyst substrate.
Common cause
6
Aftermarket-modified or deleted emissions equipment
Medium severity
Diesel pickups of this generation are commonly modified with aftermarket tuners or emissions deletes, which will directly fail the catalyst efficiency monitor.
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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light