P0420 on a 2020 Ford Expedition

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 Expedition

Ranked from 379 NHTSA owner complaints filed for this vehicle.

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1
Cam phaser/timing chain wear causing chronic rich/lean pulses that degrade the catalyst
Medium severity
This engine's documented cam phaser wear (reported independently by two owners as a cold-start rattle) can produce repeated timing-related fuel excursions that gradually push the converter below its efficiency threshold.
Reported in 2 NHTSA complaints
Parts:cam phaser timing chain
2
Catalytic converter recall/known-defect false efficiency trigger
Medium severity
Ford issued a catalytic converter-related service action for this platform; one owner reported the dealer reset the check-engine light for the recall but it re-triggered again within days.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Worn or physically failed catalytic converter
High severity
Straightforward converter wear/failure was directly diagnosed on this vehicle after the check-engine light illuminated and the vehicle jerked.
Reported in 1 NHTSA complaint
Parts:catalytic converter
4
Prolonged, uncorrected cylinder misfire overheating the catalyst
High severity
Repeated raw-fuel dumps from an active misfire can overheat and degrade the catalyst; one owner explicitly reported ongoing misfires with a total loss of power.
Reported in 1 NHTSA complaint
5
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A degraded rear O2 sensor can mimic reduced catalyst efficiency to the PCM even when the converter itself is healthy.
Common cause
Parts:oxygen sensor
6
Exhaust leak ahead of the downstream oxygen sensor
Medium severity
A leaking gasket or joint upstream of the rear sensor dilutes the exhaust sample and can trigger a false efficiency code.
Common cause
Parts:exhaust gasket exhaust manifold gasket
7
Aftermarket or non-OEM replacement converter degrading prematurely
Low severity
A lower-grade replacement unit installed during a prior repair can fall below the efficiency threshold sooner than an OEM part.
Common cause
Parts:catalytic converter

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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