Bosch Dishwasher repair

Our 3 year old Bosch dishwasher has developed the following fault:
At the end of the cycle, the pump will run continually, even with the
door open. Tried a reset, by holding down two of the programme buttons,
and this has no effect, other than the "end" LED lighting up. Pulled
the unit out to have a look at the pipes etc, and this cured the pump
problem, until the end of the next wash cycle, where the problem showed
up again. Is the pump on a "timed" cycle at the end of a wash, or is
there some sort of sensor that tells the controller when the machine is
empty of water?

Cheers