P0420 on a 2019 Ford F 150 Super Crew

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 Super Crew

Ranked from 953 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Worn/failed catalytic converter
High severity
The converter itself was diagnosed as failed and requiring replacement on this vehicle on more than one occasion.
Reported in 2 NHTSA complaints
Parts:catalytic converter
2
Oil consumption contaminating the catalyst
Low severity
Oil reaching the exhaust from the documented consumption issue on this vehicle can foul the catalyst coating.
Reported in 2 NHTSA complaints
3
Exhaust manifold leak ahead of the catalyst/sensor
Medium severity
A leaking manifold lets extra oxygen reach the downstream sensor, skewing the efficiency calculation; diagnosed on this vehicle as a faulty exhaust manifold.
Reported in 1 NHTSA complaint
Parts:exhaust manifold gasket
4
Catalyst damage from prolonged, unresolved cam-phaser-related misfires
Medium severity
Unburned fuel from the cam-phaser-driven misfire pattern documented across this vehicle's complaints (Ford App-confirmed misfire) can overheat and degrade the catalyst over time.
Reported in 1 NHTSA complaint
Parts:catalytic converter
5
Failing downstream oxygen sensor
Low severity
A sluggish or biased rear sensor can mimic a degraded-catalyst reading even with a healthy converter.
Common cause
Parts:oxygen sensor
6
General exhaust leaks (gaskets/joints)
Low severity
Any leak ahead of the rear sensor admits extra air and can trip the efficiency code.
Common cause
Parts:exhaust gasket

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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