P0420 on a 2020 Ford F 150 Supercab 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 2020 Ford F 150 Supercab Diesel

Ranked from 506 NHTSA owner complaints filed for this vehicle.

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1
Catalyst damage from chronic misfire (cam phaser/VCT failure)
Medium severity
The same 5.0L engine has a well-documented cam phaser failure pattern (rattle, rough idle, campaigns 21B10/21N03); prolonged misfire from bad valve timing sends unburned fuel into the converter and degrades it over time.
Reported in 8 NHTSA complaints
2
Oil contamination of the catalyst from excessive oil consumption
Medium severity
Multiple owners report heavy oil consumption and eventual engine replacement on this generation 5.0L; burned oil passing through the exhaust coats and poisons the catalyst substrate.
Reported in 7 NHTSA complaints
3
Worn or failed catalytic converter
High severity
Two owners had the catalytic converter directly diagnosed as failed by a dealer/independent shop after a check-engine light and loss of fuel economy -- the converter itself had lost efficiency.
Reported in 2 NHTSA complaints
Parts:catalytic converter
4
Exhaust leak ahead of the downstream sensor
Medium severity
A reported factory turbo-mounting flaw warps the exhaust manifold and breaks its mounting bolts, creating a leak ahead of the sensor that can throw off the catalyst efficiency reading.
Reported in 1 NHTSA complaint
5
Failing downstream O2/air-fuel sensor
Medium severity
A lazy or degraded downstream sensor reports readings too close to the upstream sensor, tripping a false efficiency code even with a healthy converter.
Common cause
Parts:oxygen sensor
6
Age/mileage-related catalyst wear (no other fault)
Medium severity
Catalyst substrates simply lose efficiency with mileage and heat cycles even absent any other engine fault.
Common cause
Parts:catalytic converter

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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