P0420 on a 2019 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 2019 Ford F 150 Regular Cab

Ranked from 953 NHTSA owner complaints filed for this vehicle.

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1
Chronic oil consumption contaminating the catalyst
Medium severity
Multiple owners on this truck report persistent low oil/oil consumption between changes; oil burned through worn rings/guides coats and poisons the catalyst washcoat, cutting its efficiency.
Reported in 2 NHTSA complaints
2
Repeated rough-idle/misfire episodes (cam phaser/VCT related) degrading the substrate
Medium severity
This platform's well-known cam phaser wear produces rough idling and surging on multiple trucks; unburned fuel from those events accelerates catalyst wear over time.
Reported in 2 NHTSA complaints
3
Worn or failed catalytic converter
High severity
Dealer diagnosis on at least one truck confirmed outright catalytic converter failure requiring replacement; converters lose efficiency with age/mileage and thermal stress.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Exhaust leak (manifold/gasket) ahead of the downstream sensor
Medium severity
One truck was diagnosed with an exhaust manifold issue; a leak upstream of the rear sensor introduces outside air and can either mask or mimic reduced catalyst efficiency.
Reported in 1 NHTSA complaint
Parts:exhaust manifold gasket
5
Failing downstream (rear) oxygen sensor
Medium severity
A lazy or biased rear O2 sensor reads as a below-threshold catalyst; compare front vs. rear sensor switching activity before condemning the converter.
Common cause
Parts:oxygen sensor
6
Normal high-mileage catalyst efficiency decline
Low severity
Precious-metal catalyst activity degrades gradually with age and heat cycling even absent any other 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.

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

Matching symptoms

Check Engine Light