P0420 on a 2017 Hyundai Santa Fe

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 Hyundai Santa Fe

Ranked from 943 NHTSA owner complaints filed for this vehicle.

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1
Catalytic converter damaged by chronic Theta II oil consumption
High severity
One owner explicitly states their catalytic converter 'is going bad due to engine oil consumption problems,' and many others in this batch document the same chronic Theta II oil-burning defect; burning oil coats and degrades the catalyst substrate.
Reported in 7 NHTSA complaints
Parts:catalytic converter
2
Turbocharger oil leak into the intake (2.0T variant)
Medium severity
One owner reports a turbocharger failure with drivability/power-loss symptoms; a leaking turbo can send oil into the intake tract and foul the catalyst.
Reported in 1 NHTSA complaint
Parts:turbocharger
3
Worn or aged catalytic converter
Medium severity
Normal high-mileage thermal aging reduces oxygen-storage capacity below the OBD efficiency threshold, independent of any oil issue.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Low severity
A sluggish or biased rear sensor can mimic reduced catalyst efficiency even when the converter is fine.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Low severity
A leak between the catalyst and the rear sensor lets in outside air, skewing the efficiency reading.
Common cause
Parts:exhaust gasket
6
Long-term misfire/knock overheating the catalyst
High severity
Unburned fuel from sustained knock-related misfire overheats and melts the substrate, echoing the knocking symptoms reported across this batch.
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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light