P0420 on a 2017 Ford F 350 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 2017 Ford F 350 Super Crew

Ranked from 560 NHTSA owner complaints filed for this vehicle.

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1
Oil contamination of the catalyst from engine oil consumption/leaks
Medium severity
Recurring upper-oil-pan leaks, a restricted CCV separator causing oil loss, and rod-bearing wear all point to oil reaching the exhaust stream, which coats and poisons the catalyst.
Reported in 4 NHTSA complaints
Parts:catalytic converter upper oil pan gasket crankcase ventilation (CCV) separator
2
Catalyst substrate damage from a prolonged rich/misfire condition
Medium severity
CP4 high-pressure fuel pump failures on this truck dumped unmetered fuel and metal debris into the engine, and unburned fuel passing into the exhaust during a misfire event overheats and damages the catalyst.
Reported in 2 NHTSA complaints
Parts:catalytic converter
3
Catalytic converter internal failure/overheating
High severity
One owner directly observed the catalytic converter overheating, leaking black liquid onto sensor wiring, and smoking while driving — a converter breaking down internally.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A lazy or contaminated rear O2 sensor reads efficiency incorrectly and sets the code even with a healthy catalyst.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leak before the rear sensor draws in outside air and disrupts the efficiency comparison the PCM relies on.
Common cause
6
Worn-out catalytic converter from normal high-mileage aging
Medium severity
Catalyst substrate naturally loses conversion efficiency with age and mileage until it falls below the OBD-II threshold.
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