2001 Forester 2.0 4EAT
I've had to deal with a similar error code, but mine was a vacuum leak/O2 sensor related error code, went away after fixing cracked hoses around fuel tank purge valve, the charcoal canister, and the O2 sensor.And of course to add to my blood pressure lmao, the Check Engine light came on last night(imagine you see what looks like engine oil on top of your block after just having rebuilt your engine and you get a cell) lol, which turned out to be P0440 (Evaporative Emission System).. not that big a deal apparently.
Now I'm getting the P0420, (catalyst efficiency below threshold), not worried about it though, I have no emission testing where I live.
Found something related to your issue vv
"P0440 basically means your fuel tank or related hoses and piping have a leak somewhere. Do you smell gasoline outside the vehicle at all? Probably won't if it's a small leak.
Check fuel filler pipe. On some Suby's (especially here in NY with all the salt!) the fuel filler pipe corrodes. See if there is a plastic 'cover' over your filler pipe in the wheel well. If so, remove the plastic cover and see if there's any serious corrosion of the filler pipe.
Those are the easiest things to check first. Other than that, there's a drain valve on the pack that could be having an issue. Purge control solenoid could be having issues. evap lines could be cracked. canister could be cracked or leaking. fuel tank could have a leak.
If the shop has a smoke machine that can detect small evap leaks that might be worth a shot. I've heard the smoke machines are very effective at finding leaks."