First tank, had a stick on temp gauge, and a digital one. Battery died in the digital, so was going off the tank one for awhile because I was lazy. Tank one stopped reading one day and thought "stupid cheap piece of crap". So when I finally got around to it a day or 2 later I replaced the battery's in my digital.
Turns out the stick on temp thing was right. My heater had just stuck on so it was now out of range. DOH!.