P0420 on a 2019 Ford F 150 Super Crew Diesel

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 Diesel

Ranked from 953 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Worn or failed catalytic converter
Medium severity
One owner had the dealer diagnose an outright failed catalytic converter that needed replacement after the check-engine light illuminated.
Reported in 1 NHTSA complaint
Parts:catalytic converter
2
Exhaust leak upstream of the rear O2 sensor (faulty exhaust manifold)
Medium severity
One owner's dealer diagnosed a faulty exhaust manifold needing replacement; a leak at the manifold ahead of the catalyst lets in outside air that skews the downstream O2 sensor reading and can mimic a degraded catalyst.
Reported in 1 NHTSA complaint
Parts:exhaust manifold exhaust manifold gasket
3
Failing downstream (rear) O2 sensor
Low severity
A slow or biased post-catalyst oxygen sensor can register an apparent efficiency drop even with a healthy catalyst; compare front vs rear O2 waveforms.
Common cause
Parts:oxygen sensor
4
Long-term engine misfires melting the catalyst substrate
High severity
Repeated unburned-fuel slugs from an untreated misfire condition overheat and physically damage the catalyst's substrate, permanently dropping its efficiency.
Common cause
5
Oil or coolant contamination of the catalyst
High severity
Oil consumption or a coolant leak reaching the exhaust stream coats and poisons the catalyst washcoat.
Common cause
6
Rich-running condition from a fuel system fault
Medium severity
A leaking injector or bad fuel pressure regulator pushes excess unburned fuel into the exhaust, overloading and prematurely aging the catalyst.
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.

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Matching symptoms

Check Engine Light