P0420 on a 2016 Hyundai Tucson Fuel Cell

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 2016 Hyundai Tucson Fuel Cell

Ranked from 1,536 NHTSA owner complaints filed for this vehicle.

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1
Catalytic converter fouled/damaged by the vehicle's known oil-consumption defect
High severity
Multiple owners describe excessive oil consumption (Theta II engine defect) that they or their shop directly link to catalytic converter failure/melting, including converters replaced more than once on the same vehicle.
Reported in 5 NHTSA complaints
Parts:catalytic converter
2
Prolonged/unresolved misfire melting the catalyst substrate
High severity
Owners report random multi-cylinder misfire and dead-cylinder conditions on this vehicle; sustained raw fuel passing into the exhaust overheats and damages the catalyst.
Reported in 2 NHTSA complaints
Parts:ignition coil spark plug
3
Cracked exhaust manifold/turbocharger allowing a leak ahead of the sensor
Medium severity
One owner's exhaust manifold and turbocharger were confirmed cracked by the dealer, which both leaks exhaust ahead of the monitored sensor and can throw off catalyst-efficiency readings.
Reported in 1 NHTSA complaint
4
Worn/aged catalytic converter (mileage-related)
Medium severity
Catalysts naturally lose efficiency with age and mileage; a converter at or beyond typical service life will trip a low-efficiency code even without a specific failure event.
Common cause
Parts:catalytic converter
5
Failing downstream (rear) oxygen sensor
Medium severity
A degraded rear O2 sensor can report catalyst inefficiency even when the converter itself is still good; compare upstream/downstream sensor switching activity to confirm.
Common cause
Parts:oxygen sensor
6
Exhaust leak upstream of the sensors
Low severity
A leak in the exhaust manifold or front pipe dilutes the sample seen by the sensors and can present as reduced catalyst 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.

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Matching symptoms

Check Engine Light