P0420 on a 2015 Ford Fusion Energi

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 2015 Ford Fusion Energi

Ranked from 995 NHTSA owner complaints filed for this vehicle.

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1
Catalytic converter damage from repeated cylinder misfires / coolant intrusion (1.5L EcoBoost)
High severity
This engine has a well-documented head-gasket/cylinder-wall coolant-intrusion failure; several owners report resulting cylinder misfires (P0302/P0303/P0316) and one describes a catalytic converter leak occurring alongside a coolant-intrusion event, consistent with repeated misfires or coolant washing fouling the catalyst.
Reported in 4 NHTSA complaints
2
Worn or failed catalytic converter
Medium severity
One owner had a catalyst-failure code retrieved and a rattling catalytic converter that had already been replaced once before, a straightforward end-of-life catalyst failure.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Failing downstream (rear) O2 sensor
Low severity
A degraded or slow-responding post-cat oxygen sensor can report an efficiency drop even on a healthy catalyst; confirm with live O2 sensor waveform comparison front vs rear.
Common cause
Parts:oxygen sensor
4
Exhaust leak upstream of the rear O2 sensor
Low severity
A leaking exhaust manifold gasket, flex pipe, or flange ahead of the catalyst lets outside air skew the rear O2 sensor reading; check for soot streaking at joints.
Common cause
Parts:exhaust gasket
5
Oil or coolant contamination of the catalyst substrate
High severity
Oil or coolant reaching the exhaust (burning oil, head gasket leak) coats and poisons the catalyst washcoat, permanently reducing conversion efficiency.
Common cause
6
Rich-running condition from an unrelated fuel system fault
Medium severity
A leaking injector or bad fuel pressure regulator can push excess unburned fuel into the exhaust, overloading and overheating the catalyst.
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