P0420 on a 2021 Ford Bronco Sport

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 2021 Ford Bronco Sport

Ranked from 665 NHTSA owner complaints filed for this vehicle.

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1
Catalyst fouling from cracked fuel-injector leaks (recall 25V467)
Medium severity
Raw fuel escaping cracked injectors onto the engine/into the intake stream can wash over and foul the catalyst substrate over time, on top of the recall's fire risk.
Reported in 11 NHTSA complaints
Parts:fuel injector
2
Worn/failing catalytic converter
High severity
This fleet shows documented catalytic converter fractures and warranty replacements, including one vehicle that had the converter replaced under an emissions warranty extension.
Reported in 2 NHTSA complaints
Parts:catalytic converter
3
Oil/coolant contamination of the catalyst
High severity
The documented 1.5L EcoBoost head-gasket/cracked-block defect lets coolant into the combustion chamber; coolant/oil burned through the exhaust can poison the catalyst washcoat.
Reported in 2 NHTSA complaints
4
Failing downstream (post-cat) O2 sensor
Low severity
A lazy or biased downstream sensor can mimic reduced catalyst efficiency even when the converter itself is still good.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the catalyst
Medium severity
A leak upstream of the converter dilutes the exhaust with outside air, causing both sensors to read abnormally and triggering a false efficiency code.
Common cause
6
Long-term misfires melting the catalyst substrate
High severity
Repeated unburned fuel reaching the converter (a real risk given this vehicle's injector defect) overheats and melts the substrate, permanently killing efficiency.
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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light