If your car is making squeaking noises (especially on cold start), are around the recommended service interval, or if you see any cracks or wear on your belts it may be time to replace them. This is a fairly cheap and easy fix, so if you are not sure it is best to just change it. (I like to keep an extra one in the car if I am going to very rural areas, b/c it can leave you stranded if it breaks…)
This write up was done using images from an Outback but it should be very similar to just about any 2.5l Subaru.
Start with a cool car then undo the cover for the belts, it goes between 10mm and 12mm. Extension may be required here. I’d also like to add, the bolt that holds the cover on, do not remove all the way, as it also holds the alternator in place and there is a small bracket it attaches to at the back of the alternator, that is held in place only with that bolt.
Loosen 12mm Bolt to Undo the alternator, which acts as a tensioner on the first belt. Its on the left side of the alternator and you can see a long bolt and where the alternator can move. When you’re done undoing it you should be able to simply slide the belt off.
Still using the 12mm loosen the tensioner pulley. Don’t remove it, then switch to the bolt that pulls the pulley up on the slide. Make sure it is loose before undoing the long bolt, otherwise you could snap it. Simply undo it all the way, and you should be able to remove the AC belt.
Should go in, in reverse order. Tension the belts till you have about .5 inch of play. (meaning you push on it with one or two fingers it should only move a little.) Then get the car warmed up (about 15min and tight it again till there is only .5 inch of play)
Put everything back the way you found it.
Here is an overview from the Subaru manual.