P0420 on a 2022 Hyundai Tucson

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 2022 Hyundai Tucson

Ranked from 364 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalytic converter damage from repeated untreated cylinder misfire (linked to this vehicle's GDI injector defect)
High severity
Two owners had the catalytic converter replaced (in one case twice) alongside repeated fuel injector failures — unburned fuel from the injector-driven misfires is a documented way to overheat and damage the converter.
Reported in 2 NHTSA complaints
Parts:catalytic converter
2
EGR valve fault altering exhaust composition
Low severity
One owner's EGR valve failed twice; a malfunctioning EGR can change exhaust composition enough to affect the downstream sensor's efficiency reading.
Reported in 1 NHTSA complaint
Parts:EGR valve
3
Naturally worn or degraded catalytic converter
Medium severity
Normal aging/mileage reduces oxygen storage capacity and efficiency independent of any other fault.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A slow or biased post-catalyst sensor can make a healthy converter appear inefficient to the PCM.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the sensors
Low severity
A leak ahead of the converter dilutes the exhaust sample and skews the efficiency comparison.
Common cause
Parts:exhaust gasket
6
Oil or coolant contamination of the catalyst
Medium severity
Oil or coolant reaching the exhaust stream (from an unrelated leak) can coat and poison the catalyst.
Common cause
Parts:catalytic converter

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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