Sharen, in my experience, wear and tear. Normally tear by the user. If these pumps are dropped, it's the bottom of the door that usually takes the hit. If the doors are continually removed, for cleaning etc, the hinges will weaken over time - a loose hinge usually upsets the occlusion calibration.
Also, I've had a number of situations where the butterfly cut-off, from the fluid bag, has dropped between the door and the peristaltic mechanism. Not only does this put a strain on to the door, but it also causes over infusion as it allows for a freeflow condition. Make sure that the EPROM addresses are set as per the service manual.
Cheers
Mark
