Dodger524 said:Mine only ever shows red, and then only if the float switch hasn't jammed, so the advice to look down the hole is good.
For memory (rather shaky at the best of times) when the red light illuminates there is supposed to be room for another litre. So unless you have had a curry you should be OK until morning.
As a newcomer to caravanning and the dark arts surrounding the toilet cassette. On our first trip away I expected the red light to be brighter. When I saw a rather dull red glow, I thought it was a warning that it was begining to get full. When I did do the rather sensible thing of looking down the hole, it was too late, the, hmm, cassette was *over full". Shan't make THAT mistake again..........