P0420 on a 2022 Ford F 150 Super Crew 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 2022 Ford F 150 Super Crew Diesel

Ranked from 559 NHTSA owner complaints filed for this vehicle.

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1
Catalyst damage from chronic misfire (2.7L EcoBoost intake valve failure)
Medium severity
The same intake-valve failure pattern reported on this powertrain (misfire warning, broken intake valve, catastrophic damage) sends unburned fuel through the exhaust, which degrades the catalyst over time.
Reported in 3 NHTSA complaints
2
Exhaust/catalytic converter leak
High severity
One owner had fumes leaking into the cabin from the catalytic converter area along with a separate exhaust gasket leak -- a leak at or around the converter directly undermines catalyst efficiency readings.
Reported in 1 NHTSA complaint
Parts:catalytic converter exhaust gasket
3
Failing downstream O2/air-fuel sensor
Medium severity
A lazy or degraded downstream sensor reads too similar to the upstream sensor, tripping a false efficiency code with a healthy converter.
Common cause
Parts:oxygen sensor
4
Worn or aged catalytic converter
Medium severity
Catalyst substrates lose efficiency with mileage and heat cycling even without another underlying fault.
Common cause
Parts:catalytic converter
5
Exhaust leak ahead of the sensor
Medium severity
Any leak in the exhaust upstream of the sensor introduces outside air and can throw off the efficiency calculation.
Common cause
6
Oil or coolant contamination of the catalyst
Medium severity
Oil or coolant burned through the combustion chamber can coat and poison the catalyst substrate, dropping its efficiency.
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