P0420 on a 2020 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 2020 Ford F 150 Super Crew Diesel

Ranked from 506 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Worn/failed catalytic converter
Medium severity
Three separate owners had the catalytic converter(s) diagnosed and confirmed failed by both independent shops and the dealer, one noting dual converters had both failed with a resulting drop in fuel economy.
Reported in 3 NHTSA complaints
Parts:catalytic converter
2
Repeated misfires/rough running (cam phaser and oil-consumption issues) damaging the catalyst
High severity
The same batch of trucks documents cam-phaser failures and an oil-consumption defect causing rough running; sustained unburned fuel reaching the exhaust is a well-known way to overheat and degrade a catalytic converter.
Reported in 2 NHTSA complaints
3
Failing downstream O2 sensor
Low severity
A degraded post-catalyst oxygen sensor can mimic reduced catalyst efficiency and is one of the most common non-catalyst causes of this code.
Common cause
Parts:oxygen sensor
4
Exhaust leak ahead of the downstream sensor
Medium severity
A leaking exhaust joint or gasket before the rear sensor lets in extra air and skews the efficiency reading.
Common cause
5
Oil contamination of the catalyst
High severity
Oil reaching the exhaust from worn valve seals or a failing PCV system coats and poisons the catalyst coating, dropping conversion efficiency.
Common cause
6
Loose or damaged exhaust component upstream of the converter
Low severity
A cracked manifold or loose joint changes exhaust flow and backpressure enough to false-trigger the 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.

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Matching symptoms

Check Engine Light