P0420 on a 2019 Ford F 350 Crew
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 Crew
Ranked from 463 NHTSA owner complaints filed for this vehicle.
- 1
Cracked or clogged diesel particulate filter / oxidation catalyst (DPF/DOC)
High severityOwners reported a cracked particulate/soot sensor and a related aftertreatment fault code, both pointing to DPF/DOC hardware issues that will also trip a catalyst-efficiency code.
Reported in 2 NHTSA complaints - 2
Oil leak burning onto the exhaust and fouling the catalyst
Medium severityAn upper oil pan leak reported with white smoke and a burning odor is consistent with oil reaching the exhaust stream and coating the catalyst.
Reported in 1 NHTSA complaint - 3
Fuel system contamination event damaging the aftertreatment system
Medium severityA CP4 pump failure that sent metal shavings through the injectors can also push unburned fuel/debris downstream, degrading the catalyst.
Reported in 1 NHTSA complaintParts:catalytic converter - 4
Failing downstream oxygen/NOx sensor
Low severityA degraded downstream sensor can report a false efficiency failure even with a healthy catalyst; compare sensor switching patterns.
Common causeParts:oxygen sensor - 5
Exhaust leak upstream of the downstream sensor
Low severityA leaking joint or gasket ahead of the rear sensor skews the catalyst monitor; inspect for soot streaking at joints.
Common causeParts:exhaust gasket - 6
Long-term misfires or boost loss overheating the catalyst substrate
Medium severitySustained rough running or turbo boost problems push excess unburned fuel into the exhaust, thermally damaging the catalyst over time.
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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