P0420 on a 2017 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 2017 Ford Fusion Energi

Ranked from 1,030 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalyst contamination from ongoing coolant intrusion into the cylinders
High severity
This engine has a heavily-documented coolant-intrusion issue in this batch (white/sweet exhaust smoke, coolant found in cylinders); combusted coolant/antifreeze additives foul and can destroy the catalyst washcoat.
Reported in 24 NHTSA complaints
Parts:catalytic converter
2
Catalyst damage from sustained cylinder misfires
High severity
Several complaints document diagnosed cylinder misfires on this engine; unburned fuel reaching the converter overheats and can melt the substrate.
Reported in 5 NHTSA complaints
Parts:catalytic converter
3
Worn or failed catalytic converter
High severity
Normal age/mileage degradation reduces oxygen storage capacity; confirm with a rear O2 sensor switching-rate comparison to the front sensor.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A lazy or offset rear sensor mimics a failed converter even when the catalyst itself is fine.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leak drawing outside air skews the rear sensor reading toward a false efficiency-below-threshold result.
Common cause
6
Damaged or aftermarket exhaust components
Low severity
Cracked piping or a non-OEM converter can fail the efficiency monitor even without a true catalyst fault; inspect the exhaust path.
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