P0420 on a 2017 Ford Focus Bev

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 Focus Bev

Ranked from 842 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Worn or degraded catalytic converter
Medium severity
One owner reported the catalytic converter failed outright at under 100,000 miles, well short of its expected service life.
Reported in 1 NHTSA complaint
Parts:catalytic converter
2
Oil/coolant contamination of the catalyst from a head-gasket failure
High severity
One owner's head-gasket failure (with coolant/oil cross-contamination) preceded a long-block, turbo, and catalytic-converter replacement — internal fluid intrusion is a well-known way to poison a converter.
Reported in 1 NHTSA complaint
3
Catalyst damage from a cracked cylinder/misfire event
High severity
The same long-block/turbo/catalytic-converter replacement was tied to a crack in cylinder 3, consistent with sustained misfire heat damaging the substrate.
Reported in 1 NHTSA complaint
4
Failing downstream O2 sensor
Medium severity
A lazy or biased post-cat sensor can mimic reduced catalyst efficiency even with a healthy converter.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream O2 sensor
Medium severity
A leak ahead of the sensor lets in extra oxygen and skews the efficiency calculation.
Common cause
6
Exhaust manifold or downpipe gasket leak
Medium severity
A leaking joint changes exhaust gas flow/oxygen content past the sensors.
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