P0420 on a 2020 Ford F 150 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 2020 Ford F 150 Regular Cab

Ranked from 506 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Failed catalytic converter(s)
High severity
Three separate owners had a catalytic converter (in one case both converters) directly diagnosed as failed and needing replacement by a dealer or independent shop.
Reported in 3 NHTSA complaints
Parts:catalytic converter
2
Catalyst contamination from chronic oil consumption
Medium severity
This 5.0L has a documented oil-consumption defect; oil reaching the exhaust coats and degrades the catalyst over time.
Reported in 2 NHTSA complaints
3
Long-term misfire melting the catalyst substrate
Medium severity
Cam-phaser-related misfires documented on this engine can dump unburned fuel into the converter, damaging the substrate and dropping efficiency.
Reported in 2 NHTSA complaints
4
Exhaust leak ahead of the sensors (manifold bolt failure)
Medium severity
A documented factory exhaust-manifold/turbo-mounting bolt issue on this truck causes exhaust leaks upstream of the catalyst monitors, which can trigger or contribute to an efficiency code.
Reported in 1 NHTSA complaint
Parts:exhaust manifold gasket
5
Failing downstream oxygen sensor
Low severity
A lazy or degraded rear O2 sensor can mimic a failed catalyst reading even when the converter itself is still within spec.
Common cause
Parts:oxygen sensor
6
Worn or aged catalytic converter (mileage-related)
Medium severity
Baseline wear-out cause once contamination and exhaust leaks are ruled out.
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