P0420 on a 2016 Kia Sorento

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 Kia Sorento

Ranked from 1,240 NHTSA owner complaints filed for this vehicle.

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1
Catalyst fouling from chronic oil consumption (Theta II/Lambda II GDI engine)
High severity
This engine family is widely reported to burn oil past worn piston rings/valve guides; oil ash contaminates and degrades the catalyst substrate over time.
Reported in 5 NHTSA complaints
2
Catalytic converter failure (confirmed diagnosis)
Medium severity
A dealer directly diagnosed a failed catalytic converter on this vehicle after an intermittent check-engine light; confirm with a scan-tool catalyst monitor test.
Reported in 1 NHTSA complaint
3
Worn/aged catalytic converter (normal end-of-life degradation)
Medium severity
Catalyst efficiency naturally drops with mileage; verify with upstream vs downstream O2 sensor waveform comparison.
Common cause
Parts:catalytic converter
4
Failing downstream (rear) oxygen sensor
Medium severity
A lazy or biased rear O2 sensor mimics low catalyst efficiency; check sensor response time and voltage activity.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leak lets outside air skew the rear sensor reading, making the catalyst look inefficient; inspect manifold and pipe joints for soot streaks.
Common cause
6
Catalyst damage from repeated cylinder misfires
High severity
Unburned fuel dumped into a hot converter overheats and melts the substrate; check misfire counters and spark/fuel delivery history.
Common cause
7
Physical damage to the catalytic converter (impact/road debris)
Low severity
A crushed or rattling substrate reduces surface area for the reaction; a metallic rattle on acceleration or at idle is a giveaway.
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.

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Matching symptoms

Check Engine Light