Drive belt

There are a few reasons for this. The most common are that the belt has stretch and needs replaced. The tensioner is faulty and not able to hold enough tension. You replaced the belt with a longer belt by mistake. And I am sure there are others.

https://www.2carpros.com/articles/serpentine-belt-tensioner-replacement

https://www.2carpros.com/articles/replace-serpentine-belt

I am attaching the process below on this tensioner and the process to test it to find out if the spring inside has weakened.

Please run through this and let me know what questions you have.

Thanks