P0420 on a 2020 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 2020 Hyundai Palisade

Ranked from 518 NHTSA owner complaints filed for this vehicle.

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1
Repeated internal misfires/low compression from the oil-consumption defect accelerating catalyst damage
High severity
Several complaints document critically low cylinder compression, rod-bearing failure, and an explicit engine misfire immediately preceding stalls — the kind of unburned-fuel event that damages a catalyst's substrate over time.
Reported in 3 NHTSA complaints
2
Oil burning through worn piston rings coating/contaminating the catalytic converter
High severity
One owner documents visible white smoke from the exhaust alongside the well-known Lambda II oil-consumption defect — a direct match for oil contamination fouling the catalyst.
Reported in 1 NHTSA complaint
3
Worn or degraded catalytic converter (age/mileage)
Medium severity
Catalyst efficiency naturally declines with age and thermal cycling.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A degraded rear O2 sensor can no longer confirm the catalyst is working even when it still is.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the sensor
Medium severity
A leak ahead of the downstream sensor introduces outside air that skews the catalyst efficiency reading.
Common cause
6
Long-term unresolved misfires from any source melting the substrate
High severity
Any sustained misfire condition dumps unburned fuel into the exhaust, overheating and eventually melting the catalyst's internal structure.
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