The seat slides back and forth when braking or accelerating, only by a few mm but noticable. Mine have been needing doing for some years but I lived with it. The problem doesn't affect the movement of the seat using the motors. As I said above I found the hardest part was removing the seat from the car...why are they so heavy and awkward to get a grip onI've seen a few replace these. How do you know they need replacing, daft Q maybe but what triggers the need, does the seat wobble, not slide back and forth etc ??.