Right lads

Can anyone tell me how I can check if my coolant sensor works. I have the normal focus one but also a second one in the thermostat housing where the pectel takes its read from. This one has a slight leak and I was wondering how u could check if it works. My typical mpg is 25 but I just got 29.5mpg having checked and cleaned the injector wiring and unplugged this. The car is more reluctant to start before it does, seems less laggy on boost and quiter cruising at 70mph (3000rpm in 5th) but doesn't seem to boost as hard.