A belt tensioner should be replaced if any of the following issues are noted:

1. You hear noise or feel resistance as that could be a sign of potential bearing failure.
2. The belt is not tracking correctly as that could mean a loss of tension.
3. There is sticking or notchy movement. With the belt removed, use a tool to move the tensioner through its entire range of motion. It should move smoothly from stop to stop without any movement between the tensioner body and arm.
4. There is metal to metal contact or broken/cracked tensioner stops.