P0420 on a 2016 Kia Soul

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 Soul

Ranked from 802 NHTSA owner complaints filed for this vehicle.

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1
Catalytic converter clogged/melted by chronic engine oil consumption (GDI engine oil-burn defect, recall 19V120/SC176)
High severity
Owners repeatedly report the 1.6L/2.0L GDI engine burning oil that coats and eventually melts the catalyst core, several citing the P0420 code directly and the Kia SC176/19V120 recall for this exact failure chain.
Reported in 12 NHTSA complaints
Parts:catalytic converter
2
Oil-fouled spark plugs / long-term misfire accelerating catalyst damage
High severity
Unburned oil and fuel from misfiring, oil-saturated plugs is dumped into the exhaust stream, which cooks the catalyst faster than normal driving would.
Reported in 2 NHTSA complaints
Parts:catalytic converter spark plugs
3
Recall/software remedy (KSDS update) not correcting the underlying oil-consumption defect
Medium severity
Several owners report the catalytic converter failing again after the SC176 software campaign was performed, indicating the update alone does not stop the oil intrusion that damages the converter.
Reported in 1 NHTSA complaint
4
Aged/worn catalytic converter (normal high-mileage wear)
Low severity
Catalyst efficiency naturally degrades with age and mileage independent of any defect; confirm with a cat-monitor comparison of upstream vs. downstream O2 activity.
Common cause
Parts:catalytic converter
5
Failing downstream (post-catalyst) oxygen sensor
Low severity
A lazy or biased rear O2 sensor can mimic reduced catalyst efficiency; compare switching rates to the upstream sensor before condemning the converter.
Common cause
Parts:oxygen sensor
6
Exhaust leak ahead of the downstream sensor
Low severity
A leak upstream of the rear sensor dilutes the sample with outside air and falsely triggers a catalyst-efficiency code; inspect flex-pipe and manifold joints.
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