P0420 on a 2015 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 2015 Ford F 250 Supercab
Ranked from 296 NHTSA owner complaints filed for this vehicle.
- 1
DEF reductant heater/dosing system fault (P20BA)
Medium severityA widely reported reductant (DEF) heater failure prevents proper urea dosing in cold weather, so the SCR system can't reduce NOx effectively and throws an aftertreatment-efficiency-related code.
Reported in 4 NHTSA complaintsParts:DEF reductant heater - 2
Repeated regen misfueling/rough combustion damaging the catalyst
Medium severityThe recurring rough-running/knocking regen events reported on this truck push unburned fuel into the exhaust, which can coke and degrade the catalyst over time.
Reported in 4 NHTSA complaints - 3
Failed NOx sensor (SCR aftertreatment system)
Medium severityRepeated dealer diagnoses and repeat part swaps for failed NOx sensors (the P207F pattern) are the most common trigger for an emissions-efficiency fault on this 6.7L Power Stroke's SCR system.
Reported in 3 NHTSA complaintsParts:NOx sensor - 4
Exhaust gas temperature (EGT) sensor failure
Medium severityAn inaccurate EGT reading causes the PCM to misjudge catalyst/DPF temperature and flag an efficiency fault; one owner's truck needed the EGT sensor replaced per TSB 15M02.
Reported in 1 NHTSA complaintParts:EGT sensor - 5
DEF reductant level sensor fault
Low severityA misreading level sensor can trigger related dosing/efficiency faults even with adequate DEF in the tank.
Reported in 1 NHTSA complaintParts:DEF level sensor - 6
Worn or degraded catalytic converter/DOC substrate
Medium severityNormal high-mileage wear on the diesel oxidation catalyst reduces its conversion efficiency below the threshold the PCM monitors.
Common causeParts:catalytic converter - 7
Exhaust leak ahead of the sensor
Low severityA leaking joint or gasket ahead of the monitored sensor introduces false readings that mimic reduced catalyst 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.