P0420 on a 2015 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 2015 Subaru Outback
Ranked from 505 NHTSA owner complaints filed for this vehicle.
- 1
Cracked exhaust bellows/flex pipe or manifold ahead of the downstream O2 sensor
High severitySubaru issued a recall/service campaign (WUJ-95) for this model year covering a cracking exhaust pipe bellows; multiple owners on this exact vehicle had it diagnosed by the dealer as a cracked exhaust pipe/manifold letting exhaust (and outside air) leak before the sensor, with cabin exhaust odor as the telltale sign.
Reported in 5 NHTSA complaints - 2
Catalytic converter failure/corrosion (recurring even after recall repair)
High severitySeveral owners had the catalytic converter itself replaced under the same recall/service bulletin, with at least one reporting the identical failure recurring a few years later.
Reported in 3 NHTSA complaintsParts:catalytic converter - 3
Cam carrier/valve cover oil leak dripping onto the exhaust
Medium severityA known cam-carrier seal leak on this engine drips oil toward the hot exhaust; one owner was warned this can also damage the O2 sensor over time.
Reported in 1 NHTSA complaint - 4
General catalytic converter wear/efficiency degradation
Medium severityIndependent of the recall population, converters simply age out and drop below the efficiency threshold over enough mileage.
Common causeParts:catalytic converter - 5
Failing downstream (post-catalyst) O2/air-fuel sensor
Low severityA lazy or biased downstream sensor can mimic low catalyst efficiency even with a healthy converter.
Common causeParts:oxygen sensor - 6
Prior sustained misfires overheating the catalyst substrate
High severityUnburned fuel igniting inside the converter overheats and can partially melt the honeycomb.
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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