They fail (the automatic part of them). Usually they'll work manually though. They are powered via 2 sources, so make sure the replacement has the auto feature. The first source is via the 24x7 fuse block (for the automatic function). The manual function is powered off the normal distribution block up at the helm (by the switch).
To kill power to them, I pull the 24x7 fuse, and I disconnect the 8 switch terminal behind the mirror. THere's two big quick disconnects there. There's no "fuse" for the manual operation, as right next to the switch is a dc circuit breaker.
I replaced all my bilge pumps (and I renamed them as well). The factory labels them "bow, fwd, aft". As a submariner, fwd and bow sound the same to me.
I renamed them:
Fwd
Mid
Aft
www.letsgochannelsurfing.com
Then I also replaced the float switch in the shower sump.
Ironically, everything had failed for automatic function, with the exception of the aft bilge pump, which is the only one that runs a lot.
More detail can be found here:
What's Broken On Channel Surfing