The micro switch or its wiring to the kitchen tap is defective?
After checking the wiring and concluding the tap's micro switch is the issue these are generally easy to obtain as a spare and to change out.
If you put up a photo someone here might know the model and how to change the micro switch..
You can test if it is the switch, with the tap open and getting access underneath, simply short the two wires coming to the tap. If the pump runs whilst doing this the fault is the switch.. That "shorting" the two wires is all that a properly operating switch actually does.
A common issue when damp gets into the switch over time.