P0420 on a 2019 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 2019 Hyundai Tucson

Ranked from 370 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalytic converter fouling/overheating from chronic oil consumption (Theta II GDI engine defect)
High severity
Owners report needing the catalytic converter replaced repeatedly (including annually) alongside heavy oil burning, and one complaint explicitly ties oil-consumption heat damage to the catalytic converter.
Reported in 4 NHTSA complaints
Parts:catalytic converter
2
Chronic cylinder misfire overheating and damaging the converter substrate
Medium severity
This vehicle has a documented recurring cylinder-3 misfire; unburned fuel reaching the catalyst from a misfire can overheat and melt the substrate.
Reported in 1 NHTSA complaint
3
Worn-out catalytic converter (age/mileage)
Medium severity
Catalyst efficiency naturally degrades with age and mileage until it can no longer meet the threshold the monitor checks for.
Common cause
Parts:catalytic converter
4
Failing downstream (rear) oxygen sensor
Medium severity
A degraded rear O2 sensor can report catalyst inefficiency even when the converter itself is still good.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leak ahead of the rear sensor introduces outside air that skews the efficiency comparison the ECU relies on.
Common cause
6
Oil or coolant contamination of the catalyst from an internal engine leak
Medium severity
Oil or coolant burned through the combustion chambers coats and poisons the catalyst coating, dropping its efficiency below threshold.
Common cause

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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