I have a 1999 ford expedition in the summer time I serviced the ac Now in the winter my heater doesn't work I got the thermostat changed coolant is full but it still blows cold air why

chek if the electric fan is turning on when the ac is turned on

When is the last time you had your coolant flushed? First check the knob issue. I also had that problem. However if that's not the issue you probably need your coolant flushed. I just put a heater core in my 2000 expedition. That's a 5-10 hour job by yourself. I did it with a friend. This was the 3rd heater core i put in this truck. The ford dealer didnt do the job right(1500$) the first 2 times so i did it myself. The whole dash has got to come out as well as the steering wheel needs to be dropped. There is an excellent video on youtube that shows the whole process with all the hidden bolts. Not a job for a novice. Hope this helps.

i myself had this problem it cud be your thermostat malfunctioning a clogged heater core or a leak in the cooling system or even worse a bad head gasket . does your overflow tank blow out coolant? check your oil look for water if no check your coolant over flow tank does it smell like exhaust? if oil is good and overflow tank smells fine you have eliminated bad head gasket few right.so then you check thermostat make sure its not sticking open or closed .check heater core if you have heat thats good . so best case sinario its the water pump atleast was the case with mine in summer ran water in it but water evaporates quickly as expected but as winter approached i added antifreeze right then and there the water pump started to squeel and leak antifreeze out the weep hole located on the top of water pump i replaced water pump and have not had a problem since hope this helps good luck

Make sure the cooling system is not plugged up, poor coolant flow can make your engine run hot, and a plugged heater core will not allow heat in the cabin in the winter.

sounds like you have a bad temperature door actuator. This is a motor that connects to the door and uses inputs from the sensor in itself and the heater control module to open the door from hot to cold. I does take some knowledge on recalibrating this actuator and installing it. if it isn't install correctly it could damage the new part, so just take care

Low freon for the summer, and low coolant for the winter. The heat and cooling are not related to each other. Separate systems

if it comes on but blowes cold air, or the ac still works it is the heater core, if it does not come on at all change the blower motor, make sure you check the fuses firs to make sure its not that!

Not sure where you live - if in colder regions - have you changed from a summer to a winter thermostat?

Start with Flushing out your cooling system. Flush it out with Preston Radiator flush, you have your preference of ether the super flush which takes 3-6 hours or the 10 min flush which is good if you don't have the time to flush out your cooling system.

After you have flushed out your cooling system, replace your thermostat and you should be good for another cold winter driving. What happens is that after a few years of the coolant not being serviced, the coolant crystallizes and clogs up the heater core and radiator causing poor circulation. Poor circulation will cause poor heat and or overheating issues to the engine.

Before you go through all that inspect to make sure you have the right amount of coolant in your cooling system.

Good luck and thank you for using fixya

your vacuum hose to the mixing valve is likely disconected