Just new to this and looking for a solution for my problem. 2000 Dodge Ram 318 engine. I have replaced the thermostate but the cab temperature is just warm. The hose to the heater core and coming from the heater core are the same temperature. Is there some other sensor that needs to be replaced? I have also flushed out both of the lines

 

The only thing that worked for me was to disconnect the core lines, fill with water, attach air compressor to drivers side and blow the water out (with some force, used as much psi as my nozzle would push) repeated until brownish/grey sludge stopped dripping out the discharge end.

Re-connected, Re-filled, lifted the front end, burped, burped again and burped the system again. Topped off the coolant and called it good.