P0420 on a 2018 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 2018 Ford F 250 Supercab
Ranked from 397 NHTSA owner complaints filed for this vehicle.
- 1
Catalyst loading/damage from repeated fuel-pump-contamination events
Medium severityRepeated HP fuel pump failures introduced metal debris into the fuel system and caused stalling/rough-running episodes; sustained poor combustion from a contaminated fuel system can foul and accelerate degradation of the catalyst.
Reported in 4 NHTSA complaints - 2
Failed/worn catalytic converter
High severityA dealer directly diagnosed a defective catalytic converter after an 'exhaust failure, service now' warning and reduced power on the interstate, confirming converter failure as the code's trigger on this truck.
Reported in 1 NHTSA complaintParts:catalytic converter - 3
Failing downstream (post-catalyst) oxygen sensor
Low severityA degraded or lazy downstream O2 sensor reports catalyst efficiency incorrectly even when the converter itself is healthy.
Common causeParts:oxygen sensor - 4
Exhaust leak ahead of the downstream sensor
Medium severityA leak in the exhaust before the rear O2 sensor dilutes the sample with outside air, mimicking poor catalyst efficiency.
Common cause - 5
Oil or coolant contamination of the catalyst
High severityOil or coolant reaching the exhaust stream coats the catalyst substrate and disables its ability to convert exhaust gases efficiently.
Common cause - 6
Long-term misfires overheating and melting the substrate
High severityUnburned fuel from sustained cylinder misfires ignites in the exhaust, overheating and physically damaging the catalyst's honeycomb substrate.
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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