Coolent resevoir empty.

Hi guys. When i drive on the highway for more than thirty minutes my check engine light will come on. It then turns off a few trips with normal street driving. The code i get is a P0125 Is insufficient coolant temperature. My coolant reservoir is low as well. But when i remove the radiator cap i see plenty of coolant. Would a faulty engine temperature sensor cause the vehicle to use any extra coolant and dump it because it does not need it? And my temperature gauge needle shows hot at times also. Any help is greatly appreciated. I also do not have a mechanic thumb. So any pictures of locations for recommended replacement would be appreciated.