P0420 on a 2018 Hyundai Sonata

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 2018 Hyundai Sonata

Ranked from 366 NHTSA owner complaints filed for this vehicle.

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1
Catalyst substrate damage from prolonged oil-fouled-plug misfires
High severity
Multiple owners document ongoing misfire from oil-fouled spark plugs (the known Theta II defect); sustained misfire dumps raw fuel into the exhaust and overheats/melts the converter over time.
Reported in 3 NHTSA complaints
2
Catalytic converter fouled/damaged by chronic Theta II oil consumption
High severity
Owners of this exact vehicle report burned oil being expelled through the exhaust (muffler glowing red, visible oil discharge from the tailpipe), which directly coats and damages the catalyst.
Reported in 2 NHTSA complaints
Parts:catalytic converter
3
Worn/aged catalytic converter (normal degradation)
High severity
Even without oil contamination, efficiency naturally declines with mileage until it falls below the OBD-II monitor threshold.
Common cause
Parts:catalytic converter
4
Failing downstream O2 sensor
Medium severity
A skewed rear sensor can trigger the code independent of actual converter condition; compare front/rear sensor switching patterns.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the catalyst
Medium severity
A leak ahead of the converter skews the efficiency calculation even when the cat itself is otherwise healthy.
Common cause
6
Aftermarket or non-OEM replacement converter
Low severity
A budget or universal-fit converter installed at a prior repair often can't hold OBD-II efficiency thresholds even when flowing exhaust normally.
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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light