P0420 on a 2020 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 2020 Subaru Outback

Ranked from 893 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Engine oil pan seal leak contaminating the catalytic converter (2.4L Turbo FA24)
High severity
A dealer-confirmed oil pan/RTV sealant failure on the FA24 turbo lets oil migrate rearward onto hot exhaust components including the catalytic converter, degrading its efficiency and creating a fire hazard.
Reported in 1 NHTSA complaint
Parts:oil pan gasket
2
Catalyst damage from sustained cylinder misfires
High severity
Repeated carbon-buildup-driven misfires (30-45 events/3hr documented in this batch) dump unburned fuel into the exhaust, overheating and damaging the catalyst.
Reported in 1 NHTSA complaint
3
Worn or failed catalytic converter
Medium severity
Aging substrate loses oxygen storage capacity so the rear O2 sensor tracks the front one too closely; compare front/rear O2 waveforms.
Common cause
Parts:catalytic converter
4
Failing downstream (post-cat) O2 sensor
Low severity
A lazy or biased rear sensor can mimic a dead catalyst; check sensor response time and swap-test if in doubt.
Common cause
Parts:oxygen sensor
5
Exhaust leaks ahead of the rear sensor
Low severity
A leak between the converter and the downstream sensor lets in outside air, upsetting the efficiency comparison; inspect welds and flanges.
Common cause
6
Upstream O2 sensor malfunction skewing fuel trims
Low severity
A drifting front sensor can push the engine rich/lean enough to look like reduced catalyst efficiency; compare commanded vs. actual fuel trims.
Common cause
Parts:oxygen sensor

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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