P0420 on a 2017 Hyundai Elantra Gt

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 2017 Hyundai Elantra Gt

Ranked from 732 NHTSA owner complaints filed for this vehicle.

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1
Oil consumption fouling/contaminating the catalytic converter
Medium severity
Several owners report significant oil burning and sludge, and oil ash passing through the exhaust is a known cause of catalyst contamination and efficiency loss.
Reported in 5 NHTSA complaints
Parts:catalytic converter
2
Catalytic converter damage from chronic untreated misfires (repeated ignition-coil failures)
High severity
This vehicle has a documented pattern of ignition-coil failures causing confirmed cylinder misfires; unburned fuel from repeated misfires overheats and degrades the catalyst substrate, dropping efficiency below threshold.
Reported in 3 NHTSA complaints
Parts:catalytic converter
3
Worn/aged catalytic converter (normal high-mileage substrate degradation)
Medium severity
Catalyst efficiency naturally declines with age and mileage until it can no longer meet the OBD threshold.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) O2 sensor
Medium severity
A degraded downstream sensor can mimic a switching pattern too close to the upstream sensor, reading as a lost-efficiency catalyst.
Common cause
Parts:oxygen sensor
5
Exhaust leak between the catalyst and the downstream O2 sensor
Medium severity
A leak here draws in outside air, skewing the downstream sensor reading and falsely indicating low catalyst efficiency.
Common cause
6
Engine running rich from failing fuel-system components
Medium severity
A persistently rich mixture accelerates thermal damage to the catalyst, shortening its effective life.
Common cause
Parts:fuel injector

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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