Begin by finding out which of your cliff sensors is dead. I don't have a scientific method here, but I start by checking the one that is most commonly faulty.

It's the cliff sensor that is next to the suction fan. Remove the suction fan cover, then the suction fan itself.

The reason this one is usually faulty is: If the suction fan swallows any water at all, some amount will almost certainly land within this cliff sensor, causing damage.