P0420 on a 2016 Kia Optima

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 Optima

Ranked from 748 NHTSA owner complaints filed for this vehicle.

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1
Catalyst damage from chronic cylinder misfires (Theta II engine)
High severity
This vehicle shows a recurring pattern of multiple/rough misfires tied to the documented Theta II rod-bearing defect; sustained raw fuel/unburned charge reaching the converter overheats and degrades the substrate.
Reported in 7 NHTSA complaints
Parts:catalytic converter
2
Catalyst contamination from oil ash (chronic oil-consumption defect)
Medium severity
The widely-reported Theta II oil-burning defect on this vehicle sends combusted oil ash through the exhaust, which coats and poisons the catalyst washcoat over time.
Reported in 7 NHTSA complaints
Parts:catalytic converter
3
Worn or degraded catalytic converter
Medium severity
Normal thermal aging reduces oxygen storage capacity over time until the converter can no longer meet the efficiency threshold monitored by the rear O2 sensor.
Common cause
Parts:catalytic converter
4
Failing downstream (post-cat) O2 sensor
Medium severity
A lazy or degraded rear oxygen sensor reads exhaust nearly identical to the front sensor, tripping a false efficiency code even with a good converter.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the rear O2 sensor
Low severity
A leaking manifold, flex-pipe, or gasket ahead of the converter draws in outside air, skewing the rear sensor's efficiency reading.
Common cause
6
Aftermarket or damaged exhaust component
Low severity
A previously replaced or aftermarket converter/pipe section not matched to OEM cell density can under-perform even when new.
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