It's only about 20 F here. Usually, the temp gauge is straight up after about five minuets of driving, even in cold weather. Today, I made all the way to work, about a half hour drive, and the temp gauge was only halfway between cold and normal (straight up).

Any thoughts about what's causing this?

 

Thermostat stuck open

 

So what causes a thermostat to get stuck?

 

thermostats have a bi-metallic spring that changes its force with temperature. over time the metals can fatigue etc. Also if you haven't been regularly servicing your coolant (ie drain/fresh water flush/refill every 5 years or so) corrosion can build and make the thermostat sticky. Its usually a good idea when you do a coolant refresh to also replace the thermostat and the overflow reservoir cap as part of that service.

 

Thermostat stuck open and need to be
​​​​​replace with new