Heater and temp gauge problems

Poop the cap and add while the heaters on and it's running. If the waters Low heaters act really crappy. Also, the dex-a-cool can get coagulated so you might have to flush the core. As to the gauge, change the temp sensor it's like 12 bucks and if its been running bad that might help. If that doesn't solve it. Unplug the sensor and start it it should run like cr4p and the temp gauge should be max. Or is it low. But it should be at one end of the gauge. Next short the 2prog connector and the gauge should be max and the engine running badly. Also the temp light might come on. If the car still runs good when it's unplugged. It's the body control. See if the wires to it are still good, maybe it's the plug itself. If the car reacts but the gauge doesn't it's the gauge. See if its shorted and if not the panel has to go cause the panel is one whole shebang.
But it isn't running poorly, so change the temp sensor and flush the radiator if filling it doesn't work. Hope I helped.
And remember cheap guesses first!