P0420 on a 2016 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 2016 Ford F 250 Regular Cab
Ranked from 287 NHTSA owner complaints filed for this vehicle.
- 1
Clogged diesel particulate filter/exhaust filter assembly
Medium severityComplaints document the exhaust filter assembly and DPF becoming clogged and needing replacement, restricting and degrading the aftertreatment system's monitored efficiency.
Reported in 2 NHTSA complaints - 2
Premature failure of a replacement/aftermarket catalytic converter
Medium severityAn owner reports two consecutive aftermarket converters failing within a couple thousand miles, well short of expected life, directly tripping the efficiency monitor.
Reported in 1 NHTSA complaint - 3
Natural aging/wear of the catalytic converter
Medium severityCatalyst washcoat efficiency degrades over mileage and heat cycles until it falls below the OBD threshold.
Common causeParts:catalytic converter - 4
Failing downstream oxygen/exhaust sensor
Medium severityA biased or slow downstream sensor can make an otherwise healthy catalyst appear inefficient to the PCM.
Common cause - 5
Exhaust leak upstream of the sensor
Medium severityA leak ahead of the sensor skews the readings the catalyst monitor relies on.
Common cause - 6
Incomplete/failed regeneration cycles fouling the aftertreatment system
Medium severityRepeated failed or interrupted regen events let soot and unburned fuel accumulate on the catalyst/DPF, reducing efficiency.
Common cause
Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.
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.