P0420 on a 2021 Ford Explorer
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 2021 Ford Explorer
Ranked from 402 NHTSA owner complaints filed for this vehicle.
- 1
Cracked/fractured catalytic converter (welded multi-piece design)
High severitySeveral 2021 Explorer 3.0L EcoBoost owners had the welded catalytic converter split at a seam (Ford field action 21E11); one confirmed fracture, another had a converter fault code directly tied to misfires.
Reported in 3 NHTSA complaintsParts:catalytic converter - 2
Catalyst substrate damage from an unresolved cylinder misfire
High severityOne owner's cylinder-5 misfire ran alongside a catalytic-converter fault code; the shop noted 'a bad cat will cause misfires' - running on a dead cylinder erodes the substrate.
Reported in 1 NHTSA complaint - 3
Coolant/oil contamination of the converter from an internal engine leak
Medium severityA failing water pump let antifreeze into the oil and triggered fault codes with a power loss; coolant or oil that reaches the exhaust coats and poisons the catalyst.
Reported in 1 NHTSA complaint - 4
Worn/aged catalytic converter (normal end-of-life wear)
Medium severityCatalyst washcoat efficiency naturally degrades with mileage; the single most common P0420 cause across all makes.
Common causeParts:catalytic converter - 5
Failed or lazy downstream (post-catalyst) oxygen sensor
Medium severityA downstream O2 sensor that no longer switches correctly reads efficiency as failing even on a healthy converter; a standard first diagnostic step.
Common causeParts:oxygen sensor - 6
Exhaust leak upstream of the downstream O2 sensor
Medium severityA leak in the manifold, gasket, or flex pipe ahead of the downstream sensor lets in outside air, skewing the efficiency comparison and tripping P0420.
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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