Car heat wont work. What is the cause of this? How do I fix this?

Low on antifreeze, leaking antifreeze or blend door in dash not switching from cold to hot.

does the heater work and higher rpm on the highway then blow cold in town or idle??--if so--the heater is air locked because the big gallon coolant tank over on the side is dry and or clogged. That is not an overflow for overheating--the engine draws from it and flows back as the engine rpm changes. If it is dry and or clogged THE ENGINE IS SUCKING AIR. I cleaned mine and heater works perfect even before warm--and at idle.

How do I fix this problem

im not sure how much clearer I can make it. That tank over to the side of the radiator is not an overflow overheat tank--when running the engine draws in and out from that tank. When it becomes clogged or empty or little hose is bad that feeds over below the radiator cap-- the engine then sucks air and has air in it no matter how much you fill the radiator. The radiator must be full combined with cleaning and filling the tank over to the side--by filling I mean about half way is the correct amount. Even when filling the radiator you have to use various methods such as revving the engine a bit squeezing the hoses--park on a slight incline front up--to get as much of the air out as humanly possible as a starting point.

its important to understand the issue of course could be your out of coolant--however the situation with the coolant plastic tank is not "low on coolant in the engine"--I had my issue for years and constantly was topping off which it never needed anyway and was never low but I kept forcing more in. Made zero difference about the heater blowing cold at low rpm. That tank off to the side MUST have coolant and be working correctly.

This solution fixed the low speed heat problem for my 2002 Mercury Villager sport. I blew air through the hose from the inlet of the radiator to the tank to be sure it was not clogged. Then I filled the tank to the fill line and topped off the radiator. Now my heater works at lower speeds and the van runs a little cooler. Thanks GuruZPVD2!!

thanks also if it acts up again I did an update to find a loading ramp or very steep something--and put the front wheels on it and then warm up the engine with a rag loosly over the cap area and believe it or not--alot of extra air does burp out. Then retop it and your good to go. A sure sign if there is still air in there is GOING DOWN A HILL the heater blows cold--but when going uphill it blows out hotter than hell. I know its seems unbelievable that the tilting of the vehicle makes the air go back and forth--but it definitely does.. When going down a steep hill that heater core itself is the top end highest part of all the coolant and its filled with air or at least the hose is to the heater. Revving the hell out of the engine-that also will force the water through no matter the tilt of the vehicle.. ok good luck..