The guage sender is on the thermostat housing, near the bleed screw. It tells the guage and the ecu the coolant temp and also activates the coolant temp warning light. IIRC they are only about £20 from renault, if your gonna change it youll need to stick some new type d coolant in there too..I wouldnt bother if the car runs ok, mines always done it (only when weather is cold).