zanussi 6252 draining fault?

My dishwasher seems to drain alright but then displays the flashing code for faiure to drain. I have cleaned out all the filters in the base and flushed it through several times with water and detergent. It always clears the water away but does not seem to recognise that it has done so. This happened once before, months ago but cleaning it out fixed it on that occasion, pipes seem to be clear and there does not seem to be a physical problem, just control or sensors??? can anyone help please.

Thanks

Pete