P0420 on a 2015 Subaru Forester

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 2015 Subaru Forester

Ranked from 613 NHTSA owner complaints filed for this vehicle.

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1
Excessive engine oil consumption fouling the catalyst (FB25 2.5L known defect)
High severity
Multiple owners report the same documented Subaru 2.5L oil-burning defect on this year; oil burned past worn rings coats and poisons the catalyst. One case progressed to a low-oil-pressure engine seizure.
Reported in 6 NHTSA complaints
2
Cracked or failed catalytic converter
Medium severity
A dealer-confirmed cracked catalytic converter (under warranty) with an audibly louder exhaust is documented directly on this model/year.
Reported in 1 NHTSA complaint
Parts:catalytic converter
3
Failing downstream O2 sensor from an oil leak coating the sensor
Medium severity
A cam-carrier oil leak reported on this generation is documented to have ruined the O2 sensor and a VVT solenoid, which can mimic a catalyst-efficiency fault.
Reported in 1 NHTSA complaint
Parts:oxygen sensor
4
Worn or high-mileage catalytic converter (general wear, independent of oil consumption)
Medium severity
Substrate ages with mileage regardless of other defects; confirm via downstream O2 switching-rate comparison.
Common cause
Parts:catalytic converter
5
Exhaust leak ahead of the downstream sensor
Low severity
A secondary air leak between engine and rear sensor dilutes the exhaust sample the monitor reads.
Common cause
Parts:exhaust gasket
6
Long-term misfire melting the substrate
High severity
Oil-fouled plugs or ignition issues send unburned fuel/oil into the converter, which can overheat and damage the honeycomb.
Common cause
Parts:spark plugs ignition coil

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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