P0420 on a 2019 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 2019 Subaru Outback

Ranked from 1,029 NHTSA owner complaints filed for this vehicle.

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1
Oil consumption from cam-carrier/valve-cover leaks contaminating the catalyst
High severity
This generation Outback's FB25 boxer engine has a well-documented cam-carrier sealant leak; several owners here report the leak plus rapid oil loss/burning, and oil-fouled cats are a classic driver of P0420 on these leaking boxer engines.
Reported in 5 NHTSA complaints
Parts:cam carrier gasket catalytic converter
2
Exhaust leak upstream of the downstream O2 sensor (cracked manifold/flex bellows)
High severity
Multiple owners report a cracked front exhaust bellows or manifold letting exhaust gas (and CO) escape before the catalyst, which skews the downstream sensor reading and can trip P0420.
Reported in 2 NHTSA complaints
Parts:exhaust manifold exhaust flex pipe
3
Worn/degraded catalytic converter substrate (normal aging)
Medium severity
Efficiency naturally declines with age and mileage; a substrate that has lost its coating will read as 'below threshold' on a rear O2/AFR comparison.
Common cause
Parts:catalytic converter
4
Failing or lazy downstream oxygen sensor
Medium severity
A sluggish or biased rear O2 sensor can make a healthy converter look inefficient; swapping the sensor is the standard first confirmation step.
Common cause
Parts:oxygen sensor
5
Long-term engine misfire melting the substrate
High severity
Repeated unburned fuel dumped into the exhaust overheats and can physically melt the catalyst honeycomb, permanently lowering efficiency.
Common cause
6
Vacuum or intake leak skewing fuel trims
Medium severity
An unmetered air leak drives lean fuel trims that alter the exhaust O2 signature the ECM uses to judge catalyst efficiency.
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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Matching symptoms

Check Engine Light