P0420 on a 2018 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 2018 Kia Soul

Ranked from 211 NHTSA owner complaints filed for this vehicle.

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1
Chronic oil consumption (known Kia GDI engine defect) coating the catalyst over time
Medium severity
Numerous owners document this engine consuming a quart of oil every few hundred miles; oil pulled past worn rings/seals and burned in the cylinders progressively poisons catalyst efficiency.
Reported in 5 NHTSA complaints
2
Low cylinder compression / worn piston rings feeding oil and raw fuel into the exhaust
Medium severity
Shop-confirmed low or zero compression from worn rings, documented on several of these engines, lets unburned fuel and oil reach the catalyst and accelerates its failure.
Reported in 4 NHTSA complaints
3
Rod bearing failure / engine knock
Medium severity
Several complaints describe a developing rod knock or crankshaft-related fault; the resulting rich, unstable combustion can dump extra unburned fuel into the converter.
Reported in 3 NHTSA complaints
4
Catalytic converter contamination/damage from oil reaching the exhaust
High severity
Owners describe oil being lost directly into a cylinder with resulting damage to the downstream exhaust, and a separate case of exhaust smoke with an active engine-control fault -- both consistent with oil ash fouling and damaging the catalyst.
Reported in 2 NHTSA complaints
Parts:catalytic converter
5
Worn or aged catalytic converter (mileage-related)
Medium severity
Catalysts naturally lose efficiency with age/mileage; a converter reaching end of life is the baseline cause of a P0420 on any vehicle.
Common cause
Parts:catalytic converter
6
Failing downstream (post-cat) oxygen sensor
Low severity
A lazy or degraded rear O2 sensor can mimic reduced catalyst efficiency even when the converter itself is still good; testing sensor response is a cheaper first check.
Common cause
Parts:oxygen sensor
7
Exhaust leak ahead of the downstream sensor
Low severity
A leak before the rear sensor lets in outside air, throwing off the efficiency comparison the PCM uses to set the code.
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