Is the serpentine belt worn? How do you tell?
The tensioner would show if it had an indicating mark on it but I cant find one on mine but I look at the pulleys and note if the crests of the ridges on the pulleys are shiny. If they are it means the belt is worn enough that the belt has sunk the full depth of the V grooves and is driving from the bottoms of the grooves instead of the flanks.