P0420 on a 2018 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 2018 Subaru Outback
Ranked from 805 NHTSA owner complaints filed for this vehicle.
- 1
Oil consumption/oil leaks contaminating the catalyst
High severityOwners reported drastic oil burning and repeat valve-seal oil leaks; oil ash passing through the exhaust coats and poisons the catalyst.
Reported in 2 NHTSA complaints - 2
Exhaust leak or damaged flex joint ahead of the catalyst
High severityAn owner traced a cabin exhaust-fume complaint to a leaking flex joint at the catalytic converter connection, which also throws off the efficiency reading.
Reported in 1 NHTSA complaint - 3
Worn or failing catalytic converter
High severityAge and mileage degrade the catalyst's ability to store/release oxygen, the core failure this code detects.
Common causeParts:catalytic converter - 4
Failing downstream (post-catalyst) O2 sensor
Medium severityA degraded rear sensor reads too similarly to the front sensor, mimicking low catalyst efficiency even with a healthy converter.
Common cause - 5
Long-term engine misfires degrading the substrate
High severityUnburned fuel passed into a hot converter over time melts or fouls the substrate, lowering efficiency.
Common cause - 6
Coolant or oil intrusion into the exhaust from an internal engine leak
High severityCoolant/oil reaching the combustion chamber and then the exhaust stream chemically poisons the catalyst coating.
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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