I suppose it's always possible it could have started misbehaving and giving inaccurate readings, but why now all of a sudden?
Because they do that, and usually at the most inopportune time….. It could be a number of things that could cause it, but the bottom line is, once you've eliminated everything else, it becomes time to question the source of your information…in this case, your temp gauge.
It's entirely possible there's some wonky electrical issue that's cropped up, but nothing looks out of the ordinary.
There would be no way of knowing by visual inspection alone.
One of the things I know about these factory gauges is, they aren't always accurate and I kinda doubt if Chrysler ever calibrated them for accuracy. In the older D/W series trucks (and many cars, up to about the 90s) The face of these temp gauges were printed with a "C" and an "H" at each end of the needle swing, and the word "NORMAL" with a series of hash marks across it's face. Then someone would come along and say their truck was running very warm or hot because the needle was pointing to the letter "M" or "A" when it usually pointed to the letter "O" in NORMAL…. The question I would always ask is, what is the temp when the needle is pointed to the letter M or A? And they would usually say, I don't know, but the needle isn't where it's normally at, so they conclude there much be something wrong and that their truck is running hotter than normal. That maybe so, but without knowing the exact temp the engine is running at, theres no way to conclude if it's just running warm or it's overheating.
With our Dakotas, the design of the gauges are not much different, except that there are numbers printed on the face of the gauge, instead of the word "NORMAL". With the printed numbers indicating specific temperatures, it's easy to assume that it's accurate. But we can't always be certain because we can't be certain if the gauges were ever calibrated by the factory,,,, or if they were, somehow lost that calibration via malfunction, or if the gauge is working correctly at all.
The gauge functions much like a light bulb on a dimmer switch. When you turn down the dimmer, the light bulb goes dim and vise-versa, when you want to make the bulb bright. Temp gauges, fuel level gauges and modern speedos work about the same way. The sensor is basically a rheostat and the gauge reads the amount of resistance and translates it to usable info….and sometimes they can go bad.
So start from scratch. Assume that the gauge can't be trusted at all. One of the things you can try is install a known accurate gauge, -The installation doesn't have to be permanent, it just needs to be done to confirm the reading of the factory gauge. I personally prefer Autometers because they are calibrated for accuracy. For simplicity you can use a mechanical gauge. These do not require any wiring and are fully independent of the truck. If the factory gauge is accurate, this set up would tell you.
A temp gun might not work as well, it can only read radiated temp from the surface of whatever you are pointing it at. It doesn't actually read the coolant temp inside the engine, which happens to be flowing. And it can't be used when you are driving.
Ed