P0420 on a 2021 Hyundai Palisade

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 Hyundai Palisade

Ranked from 413 NHTSA owner complaints filed for this vehicle.

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1
Catalytic converter contamination from chronic oil consumption
High severity
This 2021 Palisade population shows an extremely heavy, repeated pattern of the 3.8L engine burning multiple quarts of oil between changes (piston-ring/oil-consumption complaints); oil ash passing through the combustion chambers coats and poisons the catalyst, dropping its efficiency below the P0420 threshold well before the converter's normal life.
Reported in 15 NHTSA complaints
Parts:catalytic converter
2
Catalyst damage from a sustained cylinder misfire
High severity
One complaint documents confirmed cylinder #2 compression loss, cylinder-wall scoring and heavy combustion-chamber carbon/blow-by with active misfiring; raw unburned fuel reaching a hot converter under a prolonged misfire like this overheats and can permanently damage the substrate.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Fouled/failing fuel injectors sending unmetered fuel to the catalyst
Medium severity
A complaint describing stalling and rough running traced to a full 6-injector replacement indicates injector fouling; poorly atomized or leaking fuel that isn't fully combusted accelerates catalyst fouling.
Reported in 1 NHTSA complaint
Parts:fuel injector set
4
Worn/aged catalytic converter (normal end-of-life degradation)
Medium severity
Catalysts naturally lose efficiency with mileage and heat cycles; this is the routine, non-defect cause of a P0420 on any higher-mileage vehicle.
Common cause
Parts:catalytic converter
5
Failing downstream (post-catalyst) O2/air-fuel sensor
Low severity
A lazy or degraded downstream sensor can mimic reduced catalyst efficiency even when the converter itself is still good; compare upstream/downstream sensor waveforms before condemning the converter.
Common cause
Parts:oxygen sensor
6
Exhaust leak ahead of the downstream sensor
Low severity
A leak between the catalyst and the rear sensor lets in outside air, skewing the sensor's reading toward an apparent efficiency fault.
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