P0420 on a 2019 Ford F 350 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 2019 Ford F 350 Supercab
Ranked from 463 NHTSA owner complaints filed for this vehicle.
- 1
Fuel system contamination/injector wear degrading combustion and fouling aftertreatment
Medium severityReported CP4 pump failures sending metal debris through the injectors can cause incomplete combustion, which loads extra unburned fuel and soot onto the catalyst/DPF over time.
Reported in 2 NHTSA complaints - 2
Cracked diesel particulate filter (DPF) housing/weld
High severityAn owner had a factory weld crack on the DPF piping diagnosed via code P2002, a documented issue on 2017-2020 F250/F350 Power Strokes that directly degrades aftertreatment efficiency.
Reported in 1 NHTSA complaint - 3
Failed or degraded particulate/soot sensor
Medium severityA reported particulate sensor fault (code 22E02) at only 54,000 miles indicates a sensor used to monitor aftertreatment efficiency has failed, which can set an efficiency-related code even without a truly worn converter.
Reported in 1 NHTSA complaint - 4
Incomplete/failed DPF regeneration allowing excess soot loading
Medium severityA reported MAF-related bulletin causing the truck to 'refuse to regen' lets soot build up in the aftertreatment system, reducing measured efficiency and risking limp mode.
Reported in 1 NHTSA complaintParts:mass airflow sensor - 5
Degraded or failing NOx/downstream oxygen sensor
Medium severityA worn or slow-responding downstream sensor can misreport the aftertreatment's true efficiency, a standard cause of this code.
Common causeParts:NOx sensor - 6
Exhaust leak ahead of the aftertreatment sensor
Medium severityA leak before the monitored sensor dilutes the sample and skews the efficiency calculation, a standard cause independent of true catalyst wear.
Common cause - 7
Oil or coolant contamination of the catalyst/DPF
High severityOil or coolant entering the exhaust stream (e.g., from an internal engine leak) coats the aftertreatment media 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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