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Check the PET for your 996 but it's likely very similar if not the same. I had this issue on my 997.1. You need to get a short 15mm wrench for the bolt head in back(it's a tight space back there) and then use the 24mm wrench to tighten the front tensioner nut against it.
I apologize in advance for bumping such an old thread but I think the information posted is invaluable and should be stickied/posted in the DIY thread. I’ve taken off the serpentine belt several times and it’s really pretty easy to put back on also. However this time I was doing it no matter how much I turned the tensioner with a breaker bar I wasn’t able to loosen the tension enough to put the belt back on. I finally found this thread and tried the info that is quoted. Tightening The 15mm wrench in the back fixed my issue and I was able to put the belt back on easily! I was going crazy before this trying to figure out what I was doing wrong so this helped me a ton! Thanks all the valuable info and hopefully someone else gets the same use out of it as I did.