P0420 on a 2016 Ford F 250 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 2016 Ford F 250 Supercab

Ranked from 287 NHTSA owner complaints filed for this vehicle.

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1
Clogged or failing diesel particulate filter (DPF) affecting exhaust sensor readings
Medium severity
Multiple owners had the exhaust filter/DPF diagnosed as clogged and requiring replacement, which can alter the exhaust flow and sensor readings the catalyst-efficiency monitor relies on.
Reported in 2 NHTSA complaints
Parts:diesel particulate filter
2
Substandard aftermarket replacement catalytic converter/DOC
High severity
One owner explicitly warned that aftermarket converters (Walker 50370/73004) failed within 1,500-2,200 miles versus an expected 80,000-mile life, a documented direct cause of repeat catalyst-efficiency codes on this truck.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Oil contamination of the catalyst from an internal engine oil leak
Medium severity
One truck was found with a hole in the upper oil pan and documented 'exhaust issues' Ford couldn't correct, consistent with oil reaching the exhaust stream and fouling the catalyst.
Reported in 1 NHTSA complaint
4
Worn-out diesel oxidation catalyst (DOC)
High severity
Normal high-mileage wear reduces the catalyst's ability to convert exhaust gases; this is the baseline cause for the code.
Common cause
Parts:catalytic converter
5
Failing downstream oxygen/NOx sensor
Medium severity
A sluggish or biased downstream sensor makes even a healthy catalyst look inefficient.
Common cause
Parts:oxygen sensor
6
Exhaust leak ahead of the catalyst or downstream sensor
Medium severity
A leak upstream of the catalyst dilutes the exhaust sample the sensor reads, mimicking poor catalyst efficiency.
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