P0420 on a 2017 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 2017 Subaru Forester

Ranked from 578 NHTSA owner complaints filed for this vehicle.

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1
Cracked, rusted, or failed catalytic converter
Medium severity
Directly confirmed multiple times on this year/model: a dealer-diagnosed converter replacement, an independently confirmed failed converter, and a converter rusted through beyond repair (attributed to the plastic heat-shield trapping moisture).
Reported in 4 NHTSA complaints
Parts:catalytic converter
2
Exhaust leak ahead of the downstream sensor (cracked manifold/flex pipe, cabin exhaust smell)
Medium severity
Multiple owners report chronic exhaust leaks into the cabin from a cracked exhaust manifold bellows/flex pipe or exhaust pipe assembly; the resulting leak upstream of the rear sensor skews the catalyst-efficiency reading.
Reported in 4 NHTSA complaints
Parts:exhaust manifold flex pipe
3
Excessive engine oil consumption fouling the catalyst
High severity
The same Subaru oil-burning pattern seen on other model years is reported here as an engine needing replacement due to oil consumption, which coats and poisons the catalyst over time.
Reported in 1 NHTSA complaint
4
Cam-carrier oil leak coating engine-bay sensors/wiring
Medium severity
A cam-carrier leak documented on this generation Forester can drip oil onto downstream sensor wiring, causing intermittent readings that mimic a catalyst fault.
Reported in 1 NHTSA complaint
Parts:cam carrier seal
5
Worn or high-mileage catalytic converter (general wear)
Medium severity
Substrate ages with mileage independent of the specific defects above; confirm via downstream O2 switching-rate comparison.
Common cause
Parts:catalytic converter
6
Failing downstream oxygen sensor
Medium severity
A degraded rear O2 sensor can mimic low catalyst efficiency even with an adequate converter.
Common cause
Parts:oxygen sensor

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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