Why does your dodge ram 1500 heater not change from cold to heat?

Check the easy things first. Make sure there is enough coolant in system. check for bad thermostat being stuck in fully open position. After truck runs for about 10 to 15 minutes,gauge should move up about one third. If still blowing cold air, check upper radiator hose to see if it's hot, also check that water is flowing through system by squeezing hose and releasing it. You should feel water flowing and a squessing noise when releasing. If hose is hot and water is flowing, the door actuator my be faulty stuck in the AC (cold position. If you had a bad water pump, or loose belt, this would most likely cause your car to over heat. If you had a bad heater core, you would see water on floor on passenger side. This doesn't necessarily rule out heater core, cause It may be partially clogged not getting hot enough.