My car sat for 2 weeks at the mechanic to have the serpentine belt replaced (in well below zero temps the entire time). I get it back, turns over, runs hard for a few blocks as fluids get moving again. Got it home and there was no engine coolant in the reserve. I'm noticing that my thermostat is not moving past Cold anymore. Put in new coolant, level has stayed for 3 days. I run an average 10 miles one direction. Still blows somewhat warm air with heat on. Sorry, no codes. Thoughts?