No hot water down stairs?

Yes you have a flow sensor, to tell the boiler you want hw. different units use different type of switch, some a microswitch moved by a diaphram when hw turned on, you would be able to see actuating arm move, if it moves, switch fault, if not move diaphram fault. Others use different types of sensor, try manufacture web site, most have pdf downloads of manual, which will give you part number to get one

SD
February 2012