P0420 on a 2016 Ford Fusion

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 Fusion

Ranked from 1,659 NHTSA owner complaints filed for this vehicle.

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1
Catalytic converter contamination/damage from coolant intrusion
High severity
The same EcoBoost coolant-intrusion defect that fills these Fusions' cylinders with coolant (documented across many of these complaints) sends burned coolant downstream through the exhaust, coating and degrading the catalyst -- oil/coolant contamination is a recognized cause of reduced catalyst efficiency.
Reported in 17 NHTSA complaints
2
Catalyst wear from unresolved cylinder misfires
High severity
Owners report recurring, unresolved P0301/P0302 misfire codes on this vehicle; sustained misfires dump raw fuel into the exhaust and can overheat and damage the catalyst substrate over time.
Reported in 2 NHTSA complaints
3
Worn or aged catalytic converter
Medium severity
Catalysts naturally lose efficiency with age/mileage; a converter approaching 100k+ miles is a common baseline cause of this code even without another underlying fault.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A lazy or drifting downstream O2 sensor can mimic reduced catalyst efficiency even when the converter itself is fine.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream O2 sensor
Medium severity
A leak between the catalyst and the rear sensor lets in outside air, skewing the sensor's reading and triggering a false efficiency code.
Common cause
6
Rich-running condition overloading the catalyst
Medium severity
Excess unburned fuel from a fueling fault (leaking injector, high fuel pressure) can chemically overwhelm and prematurely wear 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