P0420 on a 2016 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 2016 Ford Explorer

Ranked from 2,432 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Coolant intrusion into cylinders contaminating the catalyst
High severity
Two owners report coolant leaking into the cylinders/cylinder walls; coolant burned through the engine and exhaust can coat and damage the catalyst substrate.
Reported in 2 NHTSA complaints
2
Oil contamination of the catalytic converter/O2 sensors from a turbo oil leak
High severity
An owner's leaking turbo was flagged by the dealer as a risk for oil contamination of the catalytic converters and O2 sensors if not addressed.
Reported in 1 NHTSA complaint
Parts:catalytic converter oxygen sensor
3
Exhaust leak (manifold/gasket) ahead of the sensor
Medium severity
An owner's exhaust-fumes-in-cabin complaint was tied by the dealer to a specific TSB, indicating an exhaust leak that can also skew the catalyst efficiency reading.
Reported in 1 NHTSA complaint
4
Worn/degraded catalytic converter (age and mileage)
Medium severity
Normal thermal aging of the catalyst over time drops conversion efficiency below the OBD threshold.
Common cause
Parts:catalytic converter
5
Failing downstream oxygen sensor
Medium severity
A degraded post-catalyst O2 sensor reports skewed switching activity that the PCM reads as low catalyst efficiency.
Common cause
Parts:oxygen sensor
6
Prior cylinder misfire loading/overheating the catalyst
Medium severity
Unburned fuel from a misfire event overheats and can melt or foul the catalyst substrate, a standard cause even without direct evidence in this batch.
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