1994 ford taurus heater blowing cold air

I have a 1994 ford taurus with 3.8 liter engine and an automatic temperature feature for the heater. This winter it is not warming up the air and is blowing cool/cold air. The engine temperature gauge was also showed the engining running hot. I thought it might be the engine thermostat and had that replaced. The engine is now running in the normal range, but the heater is still not warming up(except when driving at interstate speeds). Is there possibly an adjustment for the thermometer in the passenger compartment? Any suggestions?

First make sure that the coolant level is up where it should be. Be sure to open the radiator cap only when the engine is cold. If the coolant level is ok then you may have a problem with the air vent system doors. Most systems are controlled using the vacuum from the engine and a leak in the vacuum system can cause the vent doors to not work as they should. Also check the hoses going to the heater core and see if both sides are hot by feeling the hoses when the engine has warmed up. If they are hot then the trouble would seem to be with the vent doors somehow.

It is possible that when they replaced the thermostat they allowed some air into the system and that is your problem. The coolant may appear full when there is air in the system somewhere.