P0420 on a 2015 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 2015 Ford F 250 Regular Cab
Ranked from 296 NHTSA owner complaints filed for this vehicle.
- 1
Catalyst/DPF contamination from rough-running regen cycles
Medium severitySeveral owners describe knocking, vibrating, and heavy power loss during DPF regeneration, consistent with over-fueling events that can soot-load or thermally stress the catalyst over time.
Reported in 4 NHTSA complaints - 2
Failing NOx/downstream exhaust sensor
Medium severityOwners report repeated NOx sensor replacements tied to check-engine codes on this aftertreatment system; a degraded downstream sensor can falsely register low catalyst/converter efficiency.
Reported in 3 NHTSA complaintsParts:NOx sensor - 3
Catalytic converter/DOC failure or low efficiency
High severityA mechanic directly flagged the catalytic converter as suspect on this truck (on indefinite backorder), consistent with catalyst efficiency degradation.
Reported in 1 NHTSA complaintParts:catalytic converter - 4
Exhaust gas temperature (EGT) sensor fault
Medium severityOne owner's dealer replaced a faulty EGT sensor under a technical service bulletin; a bad EGT reading disrupts the regen/aftertreatment strategy that protects catalyst efficiency.
Reported in 1 NHTSA complaintParts:EGT sensor - 5
Exhaust leak ahead of the downstream sensor
Medium severityA leaking manifold, downpipe, or clamp joint introduces outside air that confuses the catalyst efficiency monitor.
Common causeParts:exhaust gasket - 6
Oil contamination of the DOC from turbo or crankcase-ventilation seal wear
Medium severityOil passing through a worn turbo seal or the closed crankcase ventilation system coats the diesel oxidation catalyst and reduces its measured 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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