P0420 on a 2021 Kia Seltos

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 2021 Kia Seltos

Ranked from 346 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalytic converter contamination/damage from chronic oil consumption
High severity
Burning oil from the documented piston-ring defect (recall SC336) coats and degrades the catalyst substrate over time, dropping conversion efficiency.
Reported in 8 NHTSA complaints
Parts:catalytic converter
2
Repeated stalling/rough-running events washing the catalyst with unburned fuel
Medium severity
Multiple complaints in this batch describe stalling and rough running, which can send unburned fuel into the converter and accelerate substrate degradation.
Reported in 3 NHTSA complaints
3
Worn/aged catalytic converter
Medium severity
Normal high-mileage degradation of the catalyst reduces its oxygen storage capacity below the efficiency threshold.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A lazy or degraded post-cat O2 sensor can mimic reduced catalyst efficiency even when the converter itself is fine.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Low severity
An intake of outside air ahead of the rear sensor skews its reading and can trigger a false efficiency code.
Common cause
6
Long-term misfire damaging the catalyst substrate
Low severity
Sustained misfire dumps raw fuel into the converter, overheating and melting the substrate; a standard root cause of P0420.
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