I assume its the one on the clocks with the water symbol. Mine keeps going off when i've got plenty of coolant in and the temps really low, are these sensors known to go? Mine is getting around the 171k mark so stuff is expected to start packing in. Thing is i turn the ignition off and on again and the light stays off, wierd tbh!
They do go yeah, need a new header tank but they aren't expensive.
Around £20 from i think.