P0420 on a 2018 Volkswagen Atlas

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 Volkswagen Atlas

Ranked from 455 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Head gasket failure contaminating the catalyst with coolant/oil
High severity
Two complaints on this exact vehicle document a diagnosed blown/defective head gasket and a separate cylinder-head replacement; coolant/oil passing into the exhaust is a well-known way to poison a converter's washcoat.
Reported in 2 NHTSA complaints
Parts:head gasket
2
Sustained misfire from valvetrain failure overheating the cat
High severity
This vehicle has a documented camshaft failure that stopped the engine at highway speed; unburned fuel passing through during that event is consistent with heat damage to the converter substrate.
Reported in 1 NHTSA complaint
3
Worn or failing catalytic converter
High severity
Age/mileage substrate degradation is the most common standalone cause of reduced conversion efficiency.
Common cause
Parts:catalytic converter
4
Failing downstream O2 sensor
Medium severity
A sluggish or biased post-cat sensor can mimic low catalyst efficiency even with a healthy converter.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Low severity
A leak lets outside air reach the sensor, corrupting the efficiency comparison the ECM relies on.
Common cause
6
Oil consumption fouling the catalyst
Medium severity
Chronic oil burning coats the catalyst over time and gradually reduces its conversion efficiency.
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