So for arguments sake, if the failed part is the spring.
What do you need to "fix" this easy fix.
Just purchase a new one from Subaru or are there better methods?

if by spring u mean the little metal spring on the fuel door itself, then yeh i just bought mine from subaru with the rest of the clip, i think it was $7. if its the cable with the plug thing at the end, then its around $70. if by pulling the release lever under the drivers seat pulls the white plug back from within the fuel filler cap area, then just buy the spring clip from subaru, should fix the problem.

^^^^WHS, just need to replace the spring, available from your local Subaru Dealership, they'll probably fit it for you too, if your unsure, at no charge

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i

went to fit the new cable today and found out that the bloody original cable had just fallen through lol and hadnt noticed it the first time i looked. fail... so we just pulled that back up and put it back in place, fitted the new spring clip and was good to go. all works perfect again. so that saved me hours of mucking around haha so if anyone needs a replacement cable, ive got 1 spare =)

but yeh, if the lever still pulls the cable fine, then definitely spend the few dollars on buying a new spring clip, it simply slides onto the fuel door