P0420 on a 2017 Ford F 350 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 2017 Ford F 350 Supercab

Ranked from 560 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalytic converter (DOC) failure or leak
High severity
One owner reports the catalytic converter overheating and leaking black liquid onto sensor wires with heavy smoking - direct evidence of catalyst breakdown on this truck.
Reported in 1 NHTSA complaint
Parts:catalytic converter
2
Chronic misfires from fuel injector/high-pressure pump metal contamination overheating the catalyst
Medium severity
Metal debris from a failed high-pressure pump damaging injectors and the EGR valve produces rough combustion that can overheat and degrade the downstream catalyst.
Reported in 1 NHTSA complaint
3
Oil consumption/loss (from a restricted CCV separator) fouling the catalyst
Medium severity
A documented CCV separator restriction on this engine/year range leads to engine oil loss; oil reaching the exhaust stream can foul and degrade catalyst efficiency.
Reported in 1 NHTSA complaint
4
Exhaust leak upstream of the downstream oxygen/NOx sensor
Medium severity
A leak ahead of the sensor lets outside air dilute the exhaust sample, causing a false efficiency-below-threshold reading.
Common cause
5
Failing downstream oxygen/NOx sensor
Medium severity
A degraded downstream sensor can mimic poor catalyst efficiency even when the catalyst itself is healthy.
Common cause
Parts:oxygen sensor
6
Worn/aged catalytic converter (normal wear)
Low severity
Catalyst substrate efficiency degrades with age/mileage independent of other faults.
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.

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Matching symptoms

Check Engine Light