Thank you both and I will try the screw cleaning first. I did loosen and tighten it as I read somewhere that might help but that didn't work. I also tried to get a new screw for it but apparently I need a particular screw as any old screw won't do! You would happen to know what I would need to order ...

Hi,
Thinks for reading and for any help!!
I have a coolant alarm fitted by my steering wheel which was something added to the truck and not done by the manufacturer. It's got the test push in button that if pressed sounds an alarm and flashes and theres a screw / temperature gauge into the coolant ...