P0420 on a 2016 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 2016 Hyundai Sonata

Ranked from 716 NHTSA owner complaints filed for this vehicle.

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1
Oil consumption from the known Theta II engine defect fouling/contaminating the catalyst
High severity
Numerous owners report the engine burning a quart of oil every few hundred to a thousand miles; oil pulled past worn rings/valve seals and burned in the exhaust coats and poisons the catalyst substrate, dropping efficiency below threshold.
Reported in 6 NHTSA complaints
2
Chronic cylinder misfires (documented on this platform) melting/damaging the catalyst substrate
Medium severity
Independent inspection on one vehicle found active multiple-cylinder misfire codes (P0300-series) with VVT sensor debris; unburned fuel dumped into the exhaust from ongoing misfires is a well-known way to overheat and kill a catalyst.
Reported in 2 NHTSA complaints
3
Worn/aged catalytic converter (normal high-mileage degradation)
High severity
Precious-metal washcoat activity naturally declines with age and thermal cycling; verify with a live-data pre/post-cat O2 or wideband comparison.
Common cause
Parts:catalytic converter
4
Failing downstream (post-cat) oxygen sensor
Medium severity
A lazy or biased rear O2 sensor can mimic reduced catalyst efficiency; compare rear sensor switching activity to the front sensor on a scan tool.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leaking manifold gasket or flange lets outside air reach the rear sensor, skewing the efficiency calculation; check for soot streaks and a hiss with the engine cold.
Common cause
Parts:exhaust gasket
6
Rich or lean fuel trim condition masking as catalyst inefficiency
Low severity
Sustained fuel trim faults change the oxygen content the catalyst sees; pull long/short term fuel trims before condemning the converter.
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