P0420 on a 2017 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 2017 Ford Fusion

Ranked from 1,096 NHTSA owner complaints filed for this vehicle.

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1
Catalytic converter contaminated by coolant intrusion into the cylinders ("open-deck" 1.5L/1.7L/2.0L EcoBoost block design flaw, Ford TSB 22-2229/22-2322/22-2134)
High severity
An overwhelming majority of complaints on this model year describe coolant leaking into cylinders 2/3 confirmed by borescope, with white smoke from the exhaust; burned coolant coats and poisons the catalyst.
Reported in 20 NHTSA complaints
2
Sustained cylinder misfires (P0300/P0302/P0303) from coolant-fouled cylinders melting the catalyst substrate
High severity
Multiple diagnoses cite misfire codes on cylinder 2/3 pulled at the same time as confirmed coolant intrusion, directly consistent with catalyst-damaging misfire.
Reported in 4 NHTSA complaints
Parts:catalytic converter
3
Naturally worn/aged catalytic converter
Medium severity
Efficiency degrades gradually with mileage/heat cycling even without an underlying engine defect; confirm with a rear O2 sensor waveform comparison to the upstream sensor.
Common cause
Parts:catalytic converter
4
Failing downstream (rear) oxygen sensor
Medium severity
A lazy or biased rear O2 sensor can mimic reduced catalyst efficiency; compare switching rate to the front sensor before condemning the converter.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the rear O2 sensor
Medium severity
A leak before the sensor lets in fresh air, skewing the rear sensor's reading and falsely signaling low catalyst efficiency.
Common cause
6
Ignition coil or spark plug misfire (independent of coolant intrusion)
Medium severity
Any chronic misfire dumps raw fuel into the exhaust and overheats the catalyst; check for stored/pending misfire codes unrelated to coolant.
Common cause
Parts:ignition coil spark plug

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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