P0420 on a 2019 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 2019 Ford F 150 Supercab

Ranked from 953 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalytic converter failure
High severity
Two separate owners had their catalytic converter diagnosed as failed and requiring replacement on this exact year/model.
Reported in 2 NHTSA complaints
Parts:catalytic converter
2
Oil consumption fouling/contaminating the catalyst
Medium severity
Multiple owners report significant unexplained oil consumption on this engine; oil reaching the exhaust stream degrades catalyst efficiency over time.
Reported in 2 NHTSA complaints
3
Exhaust manifold leak upstream of the sensor
Medium severity
An owner had the exhaust manifold diagnosed as faulty and needing replacement; a manifold leak lets outside air skew sensor readings and can register as reduced catalyst efficiency.
Reported in 1 NHTSA complaint
Parts:exhaust manifold
4
Chronic engine misfires (cam-phaser-related valve timing faults) overheating the catalyst
Medium severity
An owner's scan showed an explicit engine misfire alongside the well-documented 3.5L EcoBoost cam-phaser rattle on this platform; repeated misfires dump unburned fuel into the exhaust and can overheat/damage the catalyst over time.
Reported in 1 NHTSA complaint
5
Failing downstream oxygen sensor
Medium severity
A lazy or biased downstream sensor can indicate efficiency below threshold on an otherwise healthy catalyst.
Common cause
Parts:oxygen sensor
6
Worn/aged catalytic converter (normal high-mileage wear)
Low severity
Catalyst substrate efficiency naturally declines with age and mileage.
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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light