P0420 on a 2020 Ford F 150 Supercab

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

Ranked from 506 NHTSA owner complaints filed for this vehicle.

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1
Catalyst wear accelerated by chronic misfire (cam phaser failure)
Medium severity
The same trucks with repeated cam-phaser failures (cold-start rattle, rough idle, stalling) show the kind of chronic rough-running that shortens catalyst life.
Reported in 3 NHTSA complaints
2
Catalyst contamination from excessive oil consumption
Medium severity
Owners reporting high oil consumption on this engine (up to 2 quarts per 1,300 miles, one ending in engine replacement) raise the risk of oil ash coating and deactivating the catalyst.
Reported in 3 NHTSA complaints
3
Worn or failed catalytic converter
Medium severity
A dealer directly diagnosed a catalytic converter failure on one of these trucks after the check-engine light came on.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Exhaust leak upstream of the sensor (turbo/manifold mounting flaw)
Medium severity
This truck has a known factory turbo-mounting/bolt issue that warps and leaks the exhaust manifold, admitting outside air ahead of the sensors and skewing the efficiency reading.
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 reduced catalyst efficiency even with a healthy converter.
Common cause
Parts:oxygen sensor
6
Physical exhaust damage or loose heat shield
Low severity
Rattling exhaust components or a leaking joint near the sensors can trip the code independent of the catalyst itself.
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