P0420 on a 2013 Hyundai Elantra

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 2013 Hyundai Elantra

Ranked from 1,325 NHTSA owner complaints filed for this vehicle.

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1
Catalyst contamination from oil consumption/burning oil
Medium severity
One complaint describes a long-standing known issue of oil sitting and burning in the engine; oil burned through the cylinders coats and poisons the catalyst over time.
Reported in 1 NHTSA complaint
2
Catalyst damage from a chronic cylinder misfire
High severity
One complaint describes a misfiring cylinder that was serious enough to require full engine replacement; sustained misfire dumps unburned fuel into the exhaust and overheats the substrate.
Reported in 1 NHTSA complaint
Parts:ignition coil spark plug
3
Worn or aged catalytic converter
Medium severity
Baseline wear-and-age cause common to all vehicles at sufficient mileage.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) O2 sensor
Medium severity
A lazy or degraded rear sensor can read like a genuine efficiency drop.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leaking manifold gasket or cracked pipe ahead of the rear sensor skews the reading.
Common cause
Parts:exhaust gasket
6
Extended rich-running from a fuel-trim or injector fault
Medium severity
Excess unburned fuel accelerates catalyst wear.
Common cause
Parts:fuel injector
7
Degraded upstream (pre-catalyst) O2 sensor skewing fuel trim
Low severity
An aging front sensor slows to respond and biases fuel control, indirectly reducing catalyst efficiency.
Common cause
Parts:oxygen sensor
8
Aftermarket or damaged exhaust component
Low severity
A non-OEM converter or collapsed pipe section rarely meets the OEM efficiency threshold.
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.

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

Matching symptoms

Check Engine Light