P0420 on a 2013 Subaru Outback

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 2013 Subaru Outback

Ranked from 295 NHTSA owner complaints filed for this vehicle.

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1
Catalytic converter/O2 sensors fouled by chronic oil consumption (worn piston rings)
High severity
This generation's FB25 engine is documented for excessive oil burning; one owner traced repeated O2 sensor codes to eventual catalytic converter failure and replacement, and several other owners on this exact car report needing to add a quart of oil every few hundred to ~1,000 miles, the same mechanism that coats and poisons the catalyst over time.
Reported in 9 NHTSA complaints
Parts:catalytic converter
2
Failing downstream (post-catalyst) O2 sensor
Medium severity
One owner reported repeated downstream O2 sensor codes and sensor replacements on this car before the catalytic converter itself was condemned.
Reported in 1 NHTSA complaint
Parts:oxygen sensor
3
Head gasket leak allowing coolant/oil into the exhaust stream
Medium severity
A leaking head gasket lets combustion byproducts and coolant contaminate the catalyst; one owner's dealer found a head-gasket leak at 87,000 miles on this same model.
Reported in 1 NHTSA complaint
Parts:head gasket
4
General catalytic converter wear/efficiency degradation
Medium severity
Independent of the oil-consumption pattern, the converter simply ages out and drops below the efficiency threshold monitored by the PCM.
Common cause
Parts:catalytic converter
5
Exhaust leak ahead of the downstream O2 sensor
Medium severity
A leaking manifold gasket or cracked pipe upstream of the sensor dilutes the exhaust sample and mimics a failing catalyst.
Common cause
Parts:exhaust manifold gasket
6
Prior sustained misfires overheating the catalyst substrate
High severity
Unburned fuel igniting in the converter can partially melt the honeycomb; check for misfire codes preceding the P0420.
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