P0420 on a 2017 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 2017 Subaru Outback
Ranked from 917 NHTSA owner complaints filed for this vehicle.
- 1
Cam carrier/valve cover oil leak dripping onto the exhaust
Medium severityA well-documented issue on this engine generation: the cam carrier and spark-plug-tube gaskets seep oil that runs down onto the hot exhaust, smoking and risking fouling of the O2 sensors/catalyst; several owners on this exact vehicle had it diagnosed by a Subaru dealer.
Reported in 3 NHTSA complaints - 2
Catalytic converter/exhaust manifold corrosion ("accordion" section rusted through)
High severityOne owner's dealer found the catalytic converter/manifold section rusted through, causing a strong exhaust smell and an exhaust leak ahead of the downstream sensor.
Reported in 1 NHTSA complaint - 3
General catalytic converter wear/efficiency degradation
Medium severityIndependent of the corrosion/oil-leak pattern, converters age out and drop below the PCM's efficiency threshold.
Common causeParts:catalytic converter - 4
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 - 5
Exhaust leak ahead of the downstream O2 sensor (gasket/pipe joint)
Medium severityA leaking joint or gasket upstream of the sensor dilutes the exhaust sample and mimics a failing catalyst.
Common causeParts:exhaust gasket - 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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