P0420 on a 2018 Subaru Forester

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 Forester

Ranked from 489 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Exhaust leak upstream of the sensors (warped manifold / manifold gasket)
Medium severity
Owners report an exhaust manifold that warped and separated from the engine and repeated manifold gasket leaks; a leak ahead of the catalyst/sensors skews the efficiency comparison the monitor relies on.
Reported in 3 NHTSA complaints
Parts:exhaust manifold gasket exhaust manifold
2
Cracked or failed catalytic converter
High severity
One owner had a cracked catalytic converter found on visual inspection at ~92k miles, and another owner had Subaru replace two consecutive catalytic converters after sudden freeway power loss — a recurring failure pattern on this platform.
Reported in 2 NHTSA complaints
Parts:catalytic converter
3
Failing downstream (post-catalyst) oxygen sensor
Medium severity
A lazy or drifting rear O2 sensor can mimic reduced catalyst efficiency even when the converter itself is healthy.
Common cause
Parts:oxygen sensor
4
Failing upstream (pre-catalyst) oxygen sensor
Low severity
An inaccurate front sensor reading gives the PCM a bad baseline, making a healthy catalyst look inefficient by comparison.
Common cause
Parts:oxygen sensor
5
Oil or coolant contamination of the catalyst
Low severity
Oil or coolant reaching the exhaust stream from an internal engine leak can coat and poison the catalyst washcoat.
Common cause
Parts:catalytic converter
6
Long-term misfire damage to the catalyst substrate
Medium severity
Sustained raw-fuel dumping into the exhaust from an underlying misfire condition can overheat and melt the catalyst honeycomb.
Common cause
Parts:catalytic converter

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light