P0420 on a 2017 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 2017 Kia Soul

Ranked from 270 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalytic converter fouled/destroyed by chronic oil consumption (piston-ring defect)
High severity
Multiple owners describe the well-documented Gamma/Nu engine piston-ring oil-consumption defect (Kia TSB ENG222) burning oil down the cylinders and contaminating or melting the catalytic converter, several noting it explicitly caused their P0420 code.
Reported in 8 NHTSA complaints
Parts:catalytic converter
2
New/replacement engine installed without replacing an already-contaminated catalytic converter
Medium severity
Several owners had the engine replaced under recall/warranty but kept the original oil-fouled converter, which continued to trip P0420 afterward.
Reported in 1 NHTSA complaint
3
Worn/aged catalytic converter (normal high-mileage degradation)
Medium severity
Catalyst efficiency naturally declines with age/mileage until it can no longer keep exhaust emissions within the monitored threshold.
Common cause
4
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A lazy or degraded rear O2 sensor can mimic reduced catalyst efficiency even when the converter itself is still good; compare upstream/downstream sensor switching patterns.
Common cause
Parts:oxygen sensor
5
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leak between the catalyst and the rear sensor lets in outside air and skews the efficiency comparison; inspect for leaks/rust at flange joints.
Common cause
6
Long-term untreated misfires damaging the catalyst substrate
Medium severity
Repeated unburned fuel reaching the converter from ignition or fueling faults overheats and degrades the catalyst over time.
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