P0420 on a 2017 Kia Sportage

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 Kia Sportage

Ranked from 397 NHTSA owner complaints filed for this vehicle.

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1
Catalytic converter contamination/damage from chronic oil consumption
High severity
The Theta II engine's documented oil-burning defect sends oil ash through the exhaust, coating and poisoning the catalyst; owners separately reported catalytic converter replacements and this exact stored P0420 code on this model.
Reported in 4 NHTSA complaints
Parts:catalytic converter
2
Catalyst substrate damage from repeated cylinder misfires
High severity
Documented misfires and compression loss on this engine send unburned fuel into the exhaust, overheating and degrading the catalyst over time.
Reported in 3 NHTSA complaints
Parts:catalytic converter
3
Worn/aged catalytic converter
Medium severity
Catalysts naturally lose efficiency with age and mileage; this is the single most common cause of P0420 across all makes.
Common cause
Parts:catalytic converter
4
Failing downstream (post-catalyst) O2 sensor
Medium severity
A lazy or degraded rear oxygen sensor can mimic reduced catalyst efficiency even when the converter itself is healthy.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leak between the catalyst and the rear sensor lets outside air skew the sensor's reading toward an apparent efficiency fault.
Common cause
6
Coolant contamination of the catalyst
Medium severity
An internal coolant leak burning into a cylinder can coat and disable the catalyst; worth ruling out with a coolant-level and combustion-gas check.
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