P0420 on a 2017 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 2017 Hyundai Tucson Fuel Cell

Ranked from 983 NHTSA owner complaints filed for this vehicle.

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1
Oil contamination of the catalytic converter from excessive oil consumption (known 1.6L GDI engine defect)
High severity
This batch repeatedly documents heavy oil consumption on this engine family, including one complaint explicitly stating a new catalytic converter was needed because of burning oil.
Reported in 6 NHTSA complaints
Parts:catalytic converter
2
Catalyst substrate damage from prolonged unresolved cylinder misfires
High severity
Multiple complaints describe rough running/misfiring and a confirmed no-compression condition on this engine; raw fuel from ongoing misfires overheats and degrades the catalyst.
Reported in 4 NHTSA complaints
Parts:catalytic converter
3
Failing downstream (post-catalyst) O2 sensor
Medium severity
A lazy or degraded downstream sensor can mimic reduced catalyst efficiency even when the converter is otherwise fine.
Common cause
Parts:oxygen sensor
4
Exhaust leak ahead of the downstream sensor
Medium severity
A leak (e.g., at the turbocharger or a corroded joint) between the catalyst and the rear sensor lets in outside air and skews the efficiency comparison.
Common cause
5
Worn/aged catalytic converter
Medium severity
Normal end-of-life degradation of the catalyst washcoat reduces conversion efficiency over time regardless of other faults.
Common cause
Parts:catalytic converter
6
Upstream (pre-catalyst) O2/air-fuel sensor drift
Low severity
A skewed front sensor can bias the fuel trims enough that the catalyst appears to be underperforming on the efficiency monitor.
Common cause
Parts:oxygen sensor

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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