P0420 on a 2020 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 2020 Ford Explorer

Ranked from 1,171 NHTSA owner complaints filed for this vehicle.

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1
Cracked or fractured catalytic converter
High severity
The dominant, well-documented failure on this exact vehicle/engine — Ford's Customer Satisfaction Program 21B35 covers cracked RH/LH catalytic converters on 2020-2022 3.0L Explorers, repeatedly confirmed here by independent mechanics and dealers.
Reported in 6 NHTSA complaints
Parts:catalytic converter
2
Exhaust flex-pipe/downpipe leak
Medium severity
The same 21B35 program also covers failed exhaust flex pipes; a leak here (before or around the catalyst) skews the O2 readings the catalyst monitor depends on.
Reported in 3 NHTSA complaints
Parts:exhaust flex pipe
3
Oil or coolant contamination of the catalyst
High severity
One owner was told of possible engine coolant in cylinders, a known EcoBoost issue — coolant/oil reaching the exhaust stream poisons the catalyst's active coating.
Reported in 1 NHTSA complaint
4
Failing downstream (post-cat) O2 sensor
Medium severity
A biased or slow rear O2 sensor can make a healthy catalyst appear inefficient to the PCM.
Common cause
Parts:oxygen sensor
5
Worn/aged catalytic converter (mileage-related)
Medium severity
Independent of the known defect, catalyst efficiency naturally declines with age and mileage.
Common cause
Parts:catalytic converter
6
Long-term misfires overheating and melting the substrate
High severity
Unburned fuel from a chronic misfire ignites in the exhaust and can physically damage the catalyst's substrate.
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