Red light on Thetford toilet?

The red light on our toilet stays on even though it's been emptied and flushing procedure carried out. Any ideas ?
BP, July 2016


done
Probably need a new reed switch

Ian Naismith, December 2016
The float has a magnet that a sensor detects when chamber is full, open up the unit and look at the float

Roger, July 2016
link Click here to see other fixes for Thetford.