P0420 on a 2019 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 2019 Kia Sorento

Ranked from 418 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalyst fouling from chronic oil consumption (Theta II / Lambda II defect)
High severity
Numerous owners of this exact vehicle report heavy oil burning with visible exhaust smoke and soot, which coats and poisons the catalyst and drops its efficiency below threshold.
Reported in 6 NHTSA complaints
Parts:catalytic converter
2
Chronic cylinder misfires (rod-bearing wear/burnt valves) overheating and melting the catalyst substrate
High severity
The same engine defect that causes cylinder misfires on this vehicle sends unburned fuel into the exhaust, which can overheat and damage the catalyst core.
Reported in 4 NHTSA complaints
Parts:catalytic converter
3
Worn or aged catalytic converter
Medium severity
Normal high-mileage catalyst aging reduces oxygen storage capacity below the efficiency threshold.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A lazy or biased rear sensor can mimic poor catalyst efficiency even with a healthy converter.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the catalyst or sensors
Medium severity
A leak lets in outside air that skews the sensor signals the ECU uses to judge catalyst efficiency.
Common cause
Parts:exhaust gasket
6
Failing upstream air-fuel ratio sensor
Medium severity
A drifting front sensor can feed the ECU a bad baseline, making a healthy catalyst look inefficient.
Common cause
Parts:air-fuel ratio sensor

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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