As Rob-T says, but your van may not have microswitches on the taps, but a pressure switch in the line (although i think it more likely to be micro switches).
Look in the lockers under the taps. If your taps have microswitches, there will be two wires coming down from each tap and joining the main wiring harness of the van. Hopefully these will have connectors that you can unfasten - you only need to do one on each tap. Do this with each tap in turn and you should discover which tap is faulty.
once identified, you can get spare switches for most taps, but you may find that the switch has got a bit "gunged up" to use the technical term and that cleaning will solve the problem. Changing microswitches can be a bit fiddly depending on your level of DIY expertise.
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