P0420 on a 2016 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 2016 Subaru Outback
Ranked from 639 NHTSA owner complaints filed for this vehicle.
- 1
Exhaust leak from cracked front exhaust pipe bellows/manifold (upstream of catalyst)
High severitySubaru's own WUJ-95/WUJ-95R service bulletin describes the 2015-2016 Outback 2.5L front exhaust pipe bellows cracking near the manifold-to-catalyst junction; owners report exhaust odor/growl in the cabin, and an unmetered exhaust leak ahead of the catalyst is a textbook P0420 trigger.
Reported in 14 NHTSA complaints - 2
Secondary catalytic converter damage from prolonged upstream exhaust leak
High severitySeveral owners report the shop replaced the catalytic converter itself once the cracked bellows leak had gone unaddressed, consistent with a leak-damaged or contaminated cat throwing efficiency codes.
Reported in 3 NHTSA complaintsParts:catalytic converter - 3
Oil consumption fouling the catalyst
Medium severityOne owner documented needing to add a quart of oil every ~3,300 miles; oil burned through the combustion chamber can coat and poison the catalyst, lowering its efficiency.
Reported in 1 NHTSA complaint - 4
Worn/aged catalytic converter (normal high-mileage degradation)
Medium severityCat substrates lose efficiency with age/mileage independent of any leak; a converter simply out of range on its own is the baseline P0420 cause.
Common causeParts:catalytic converter - 5
Failing downstream (post-catalyst) oxygen sensor
Medium severityA lazy or drifting downstream O2 sensor reads too similar to the upstream sensor, tripping a false efficiency code even with a healthy converter.
Common causeParts:oxygen sensor - 6
Long-term uncorrected misfires overheating/melting the catalyst substrate
High severitySustained cylinder misfires dump raw unburned fuel into the exhaust, which combusts inside the converter and can melt the substrate, permanently killing its efficiency.
Common cause
Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.
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.