Just replaced Thermostat in our 2008 GT, due to "Coolant temp out of spec" code. Following thermostat replacement, ran the engine about 30 minutes and added a total of about 1.5 gallons of 50/50 coolant. Top radiator hose is hot, temp gauge is pointing about 4:00 where it normally does (since we bought the car about 2k miles ago), heater hoses are: One is warm and the other cold.
So it looks like no flow through the heater box. The heat was fine prior to changing the Thermostat. I checked the heat specifically wondering just how "out of spec" the Coolant temp was, and it seemed not much as the heat was blowing hot.
What should I check? How would changing the Thermostat and replacing the fluid trigger this?
Thanks for any suggestions

 

Nevermind. When I drove it, it immediately started producing heat and the Coolant level dropped. Must have been more air in the system. I had revved it a couple of times when I was adding Coolant, but apparently not enough. Problem solved.