I have a problem with my truck that I need help with: The temperature gauge slowly heats up after I start the truck(normal). As I drive the truck around, the gauge goes past normal and on to hot. Then, after about 30 seconds, the gauge goes back to normal. The engine doesn't appear to be overheated and the gauge only does this when the motor is warming up for the first time that day. Could this be a sensor or a thermostat?