P0420 on a 2019 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 2019 Ford Explorer

Ranked from 285 NHTSA owner complaints filed for this vehicle.

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1
Coolant intrusion into the cylinders contaminating the exhaust/catalyst
High severity
Two reports of coolant found in the combustion chamber/valve area with no external leak, requiring engine replacement -- contamination of this kind fouls the downstream catalyst.
Reported in 2 NHTSA complaints
Parts:water pump
2
Oil contamination of the catalytic converter from supercharger/turbo seal failure (3.0L EcoBoost twin-turbo)
High severity
Owner's mechanic diagnosed both superchargers failed, sending oil into the exhaust and producing white smoke; this is tied to NHTSA Campaign 24V635000 on this engine/model-year and is a direct route to catalyst poisoning.
Reported in 1 NHTSA complaint
Parts:supercharger catalytic converter
3
Persistent cylinder misfire (P0301) fouling/damaging the converter
Medium severity
Owner reports a stored P0301 misfire code with no clear root cause found; sustained misfires dump unburned fuel into the converter, which can overheat and kill the substrate.
Reported in 1 NHTSA complaint
4
Worn/degraded catalytic converter (age/mileage)
Medium severity
Normal substrate wear over time and mileage until it drops below the OBD-II efficiency threshold.
Common cause
Parts:catalytic converter
5
Failing downstream O2 sensor
Low severity
A sluggish or drifted downstream sensor reads too similarly to the upstream sensor and falsely reports poor catalyst efficiency.
Common cause
Parts:oxygen sensor
6
Exhaust leak upstream of the downstream sensor
Medium severity
A leak before the rear sensor introduces outside air and skews the efficiency comparison the PCM relies on.
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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light