P0420 on a 2017 Ford Fusion Hev

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 Hev

Ranked from 1,081 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalytic converter contaminated/damaged by coolant intrusion
High severity
Numerous owners report coolant burning in the cylinders and exiting as white/sweet-smelling exhaust smoke — coolant/antifreeze passing through the exhaust stream is a well-known catalyst poison and directly consistent with a P0420 catalyst-efficiency failure on this generation's known head-gasket/block issue.
Reported in 7 NHTSA complaints
Parts:catalytic converter
2
Catalyst substrate degraded by prolonged cylinder misfire
Medium severity
Unburned fuel from ongoing misfire (documented extensively in this batch) overheats and can melt the catalyst substrate over time.
Reported in 3 NHTSA complaints
Parts:catalytic converter
3
Worn/aged catalytic converter (normal end-of-life)
Medium severity
Catalysts naturally lose efficiency with age and mileage even without a triggering fault; a scan-tool O2 sensor switching test can confirm.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) O2 sensor
Medium severity
A lazy or failed 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
Low severity
A leak that draws in outside air before the sensor skews the reading and can falsely trigger a catalyst-efficiency code.
Common cause
6
Oil contamination of the catalyst
Medium severity
Oil consumption or leakage into the exhaust stream can poison the catalyst coating and reduce its measured efficiency.
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