P0420 on a 2017 Ford F 250 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 2017 Ford F 250 Regular Cab

Ranked from 1,155 NHTSA owner complaints filed for this vehicle.

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1
Failing NOx sensor (aftertreatment efficiency monitoring sensor)
High severity
This is a widely reported failure on this exact truck: a Ford TSB describes wiring-harness corrosion causing the NOx sensor to fail and throw a related code, with owners reporting the same sensor failing repeatedly around 43,000 miles.
Reported in 2 NHTSA complaints
Parts:NOx sensor
2
Faulty EGR valve affecting exhaust composition
Medium severity
Dealer diagnosis on this vehicle found the EGR valve faulty, which alters exhaust gas composition/flow and can affect aftertreatment efficiency readings.
Reported in 1 NHTSA complaint
Parts:EGR valve
3
Oil or coolant contamination of the catalyst/aftertreatment system
Medium severity
This vehicle experienced multiple oil pan failures and a coolant leak at under 138,000 miles, fluids that can contaminate downstream exhaust aftertreatment components.
Reported in 1 NHTSA complaint
4
Worn/failing catalytic converter or aftertreatment catalyst (mileage-related)
Medium severity
Catalysts and SCR/DPF aftertreatment components lose efficiency with age and mileage.
Common cause
Parts:catalytic converter
5
Exhaust leak upstream of the sensor
Low severity
A leak in the exhaust or EGR plumbing ahead of the sensor dilutes the sample and can present as reduced efficiency.
Common cause
6
Long-term misfire or incomplete combustion damaging the aftertreatment system
Medium severity
Sustained unburned fuel reaching the exhaust overheats and degrades catalyst/DPF/SCR components over time.
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