Have a C4 Thetford Bench Style Cassette Toilet in our Sprinter Campervan. The pump failed to flush when the round flush button was pushed down, so I replaced the pump as per a previous Thread on this forum. If the pump was not used once every 6 weeks the pump rotor would seize and I would need to remove the pump and use a small screwdriver and turn the impeller to free up and all is well for a while.  This routine did not work so the pump was replaced.

After replacement the pump still did not work Oh Bother.

Removed the cassette and pump fuse and replaced it with a double fuse and attached wire ( see below) to see if battery was getting to the fuse No 12v to the fuse, on further investigation I found the wire to the fuse was not connected (broken). The fuse is connected to a micro switch that confirms that a cassette is mounted in the toilet,  its in the cassette cavity on the bottom of the round knob that you turn to allow waste into the cassette. So, every time you empty the bowl of the toilet you are putting the wire that joins the fuse under strain.. its just a matter of time B4 it breaks and cuts off power to the pump..

Solution: The broken wire to the fuse was connected to the wire that went to the micro switch, under the round knob that you turn to open the cassette for waste and push the knob down ( the push down operates the micro switch) to supply power to the pump. I installed an inline fuse on the pump power lead at the battery.

Why Thetford installed a micro switch, to stop the pump operating if there was no cassette installed when there was nothing to stop the slide being opened (well B4 the flush was needed) and waste deposited into the empty chamber. Having said the above, the pump could still be operaed inadvertingly without the casssett in situ so it could be helpful in advoiding a clean up. And why have a wire to the fuse being bent every time the toilet is used. ( Note this model has been discontinued)

https://www.thetford.com/us/thetford-support/c4-cassette-toilet/

https://www.jaycar.com.au/double-standard-blade-fuse-socket-wire-tap/p/SF5115