P0420 on a 2018 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 2018 Ford Explorer

Ranked from 713 NHTSA owner complaints filed for this vehicle.

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1
Internal water pump failure sending coolant/oil into the cylinders, contaminating the catalytic converter
High severity
The 3.5L V6's internally-mounted water pump is a well-documented failure point on this vehicle; multiple owners had coolant or oil intrusion into the cylinders (in one case following a confirmed P0303 misfire), and burnt coolant/oil passing through the exhaust degrades catalyst efficiency.
Reported in 6 NHTSA complaints
Parts:water pump catalytic converter
2
Worn or failing catalytic converter
High severity
One owner's independent shop diagnosed stored code P0430 (Catalyst System Efficiency Below Threshold, Bank 2) on this same vehicle and replaced the converter.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Failing downstream oxygen/air-fuel sensor
Medium severity
A degraded rear sensor can mimic a worn converter by reporting readings too close to the upstream sensor.
Common cause
Parts:oxygen sensor
4
Exhaust leak upstream of the downstream sensor
Medium severity
A leaking joint or gasket ahead of the sensor introduces extra oxygen, mimicking reduced catalyst efficiency.
Common cause
5
Chronic multi-cylinder misfires overheating and damaging the catalyst substrate
High severity
Unburned fuel dumped into the exhaust from sustained misfires can overheat and degrade the catalyst over time.
Common cause
6
Damaged exhaust manifold or downpipe gasket
Low severity
A leaking gasket ahead of the sensor has the same effect on readings as a failing converter.
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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Matching symptoms

Check Engine Light