P0420 on a 2019 Ford F 150 Supercab Diesel
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 150 Supercab Diesel
Ranked from 953 NHTSA owner complaints filed for this vehicle.
- 1
Worn or failed catalytic converter
High severityTwo owners were told directly by the dealer that their catalytic converter had failed and needed replacement.
Reported in 2 NHTSA complaintsParts:catalytic converter - 2
Oil consumption contaminating the catalyst
High severityOwners with confirmed high oil consumption on this engine can pass oil ash through the exhaust, poisoning the catalyst coating.
Reported in 2 NHTSA complaints - 3
Exhaust manifold leak upstream of the sensor
Medium severityA faulty exhaust manifold needing replacement was diagnosed on one truck; a leak there skews the catalyst-efficiency comparison.
Reported in 1 NHTSA complaint - 4
Failing downstream (post-catalyst) O2 sensor
Medium severityA degraded rear sensor can mimic reduced catalyst efficiency even with a healthy converter.
Common cause - 5
Long-term rough running/misfires degrading the substrate
High severityUnburned fuel repeatedly reaching a hot converter damages the honeycomb substrate over time.
Common cause - 6
Oil or coolant intrusion into the exhaust stream
High severityAn internal leak that lets oil or coolant reach the exhaust chemically poisons the catalyst.
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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