What happens if a thermostat sticks open? Car.

Dash gauges are notoriously inaccurate but if yours is getting to half way, that's about normal. My gauge on my '88 Grand Caravan only gets to 1/4 scale in the winter but the heater can burn me out if I turn it all the way up. Your first clue is the heater will blow cold air for a long time, then it may eventually become slightly warm. Feel the heater or radiator hoses after the engine is warmed up. They should be too hot to hold onto for very long. If they are not, squeeze the upper radiator hose closed for a minute or two, then see if it gets hotter when you release it.