P0420 on a 2021 Kia Telluride

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 Telluride

Ranked from 337 NHTSA owner complaints filed for this vehicle.

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1
Catalyst fouling from chronic oil consumption
Medium severity
This engine is well documented in this batch for burning excessive oil between changes; oil ash carried into the exhaust coats and degrades the catalyst's conversion efficiency over time.
Reported in 3 NHTSA complaints
2
Catalyst damage from prior misfire/rough-running events
High severity
One owner's engine had a cylinder go inoperative from oil starvation; raw fuel or oil dumped into the exhaust during an event like this can overheat and damage the catalyst brick.
Reported in 1 NHTSA complaint
3
Worn-out catalytic converter
High severity
Normal high-mileage catalyst wear is the baseline cause of an efficiency-below-threshold code.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A sluggish or biased post-catalyst sensor makes even a healthy converter appear inefficient.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream sensor
Medium severity
A leak ahead of the sensor dilutes the exhaust sample it reads, mimicking poor catalyst performance.
Common cause
6
Aftermarket or substandard replacement converter
Medium severity
A replacement converter that doesn't match OEM cell density/catalyst loading can pass initial testing but fail the efficiency monitor quickly.
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