P0420 on a 2013 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 2013 Subaru Forester

Ranked from 77 NHTSA owner complaints filed for this vehicle.

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1
Excessive engine oil consumption fouling the catalytic converter (FB25 2.5L known defect)
High severity
An overwhelming share of this vehicle's complaints describe the well-documented Subaru 2.5L oil-consumption defect (class-action litigation); oil burned past worn piston rings coats and poisons the catalyst, dropping its efficiency and setting P0420. Fix requires a piston-ring/short-block repair, not a simple part swap.
Reported in 13 NHTSA complaints
2
Blown head gasket letting coolant/combustion gases cross-contaminate the exhaust
High severity
Multiple owners report a blown or suspected head gasket with oil/coolant cross-contamination (oil in the coolant tank, water in the oil); coolant reaching the catalyst is a direct cause of efficiency-code failure.
Reported in 4 NHTSA complaints
Parts:head gasket
3
Worn or high-mileage catalytic converter
Medium severity
Independent of the oil-consumption issue, normal substrate aging also reduces efficiency over time/mileage.
Common cause
Parts:catalytic converter
4
Failing downstream oxygen sensor
Medium severity
A degraded rear O2 sensor can mimic low catalyst efficiency even on an otherwise adequate converter.
Common cause
Parts:oxygen sensor
5
Long-term misfire (oil-fouled spark plugs) melting the substrate
High severity
Oil pulled past worn rings can foul spark plugs and cause misfires; unburned fuel/oil overheats and can physically melt the cat's honeycomb.
Common cause
Parts:spark plugs
6
Exhaust leak ahead of the downstream sensor
Low severity
Any secondary air leak between the engine and the rear O2 sensor dilutes the exhaust sample and can trigger the code.
Common cause
Parts:exhaust gasket

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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