The temp gauge sits all the way at the bottom and doesn't move, and the digital reads ---. Is that a sensor problem or does it not work if the thermostat is sticking?

 

Change your T stat. it is stuck open.
Same prob as this Thd https://www.chevyhhr.net/forums/problems-service-repairs-42/temp-guage-flat-coolant-temp-21362/ and has been covered many times.

How to do it yourself https://www.chevyhhr.net/forums/how-tutorial-library-21/how-save-%24350-00-thermostat-replacement-2-4l-p0128-24493/

 

Covered under Powertrain warranty if that still applies to you. My 2008 has 43k and I just had this done. The hardest thing is getting the radiator pet kock out if you dont have a way to get the car off of the ground. I let the dealer fix it.