Why does my heater fan only blow cold air?

If both heater hoses under the hood are hot, the blend door in the hvac plenum is not moving to direct airflow through the heater core. If one is hot and one is not, you have a plugged heater core. If both are cold, you have an air lock in the heater core. Check the coolant level and run veh at 2000 rpm in park with heater on full till you get heat, then top up coolant again. If this continues to happen you are getting air in the cooling system. Only comes from one place -- the head gasket(s) have failed and you are getting combustion pressure pushed into cooling system.

You have a heater conrol valve that directs hot water to the heater core. If that valve is faulty it would cause your problem. I would first inspect all the hoses under the hood to ensure that a previous owner has not disconnected the heater core. Also inspect coolant level as low coolant level will effect proper heater function.

I have had this same problem. My car will blow hot air for awhile, then suddenly it will be stone cold. I have had it checked by 2 different garages (one was the dealership). The first garage identified a problem with the rheostat (sp?) which was truly broken, so they put in a new one. Due to the on and off again nature of the problem, this seemed to fix it. When it started blowing cold again, they referred me to the dealership, where they replaced the thermostat (which the first garage had said was okay, but the dealership showed me it wasn't working). The first garage is the guy I trust, and he told me if it wasn't the part he fixed, it was probably the diverter up under the dash and he referred me to the dealership to have that fixed. I think the dealership messed with the thermostat (which worked fine at the first garage) and just replaced that, since it was quick and easy to do and would also be the obvious place for the problem to be. What I get sick of, is being treated like a "girl" who has to take them at their word. I asked to see the non-functional thermostat, but they already had it off the car, so how do I know that was even my thermostat? I asked to see the manager of the service department, and he assured me his staff is honest. Yeah, whatever. The car was blowing hot air at that point so I paid the bill and left. That was early spring, I haven't used the heat again until now and it is STILL DOING THE SAME THING it was doing before I had ANYTHING fixed. I am frustrated, and don't know who to take it to. My father was a career auto mechanic, so I grew up around this stuff and find it infuriating to be treated like a stupid woman who doesn't understand the difference between an diverter, a heater core, and a thermostat. Grrrrr.

11/15/2011 would like to know what you found. I'm now having the same pblm I have not used the heater since last winter...worked fine then. Turned it on for the 1st time this fall season....nothing but cold air. coincidentally the A/C on my vehicle wld not trun off this past Aug. It was repaired/wks good. now 1st time use of Heater. I dont like the where this is going. do not know if these 2 wk independently of each other or if there is a common link.

I had a problem like this w/ my '01 Gr. Mq. Would say first that your vent system air diversion door is not operating properly b/c the vacuum-operated door motor is bad or the vacuum hoses are not all connected, or not properly connected. My vacuum motor went bad and would only open part-way. I first thought it couldn't be the vacuum motor b/c I could hear the door moving when I started the engine. This is the door that deflects air across the under-dash heater coils so you get heated air (or not). So, if the door doesn't move to deflect air across the heater coils you get little or no heat into the passenger compartment. The second possibility may be the most probably if you get no heat whatsoever b/c that means the hot water diversion mechanism (out under the hood) is not functioning, hence hot water is not even reaching the under dash heater coils in the first place. I purchased a vacuum motor at the local Ford dealership ($25 or so)and easily replaced it myself and I'm sure you could do the same with the hot water diversion mechanism under the hood.

How do you determine if its the vacuum motor or a broken door itself?

how come i hear my a/c pump clicking while the heater is blowing. heater works fine just clicking is anoying .my a/c is out of freon but the heater and a/c shouldn't be running at the same time right?

i have a 97 ford explorer xlt 5.0 and tried the door blend and both were cold air no matter what setting the heat was on so i disconnected the ac power under the hood, and it works great so try that bc your ac may be constantly running and if u do the door blend make sure the door is shut for heat :) hope i helped

The A/C compressor will kick on when you engage the defrost to keep the compressor from seizing throughout the winter.

Having the same problem but head gasket is ok, thermostat is ok no water in the oil but when I press on the Gas it blows hot when I let off cold again

SAME issue! I have a 200 merc mountaineer. This is the second time this season. I replaced the following the first time, rad cdp, thermostat, heater core, then finally the control module <-- that was the issue the entire time. So this time i started there, still blowing cold air. My heat was working, i noticed my cap went bead again and i was low on coolent so i topped it off and put a new cap on it. Then My heat went out.. I need help!'

I figured mine out I ran water through the heater core hoses and noticed they were clear, on alot of ford's there is no valve to let the air out. So I ran my truck until I knew the thermostat was open with the radiator cap off , I applied the gas pedal and of course it started blowing antifreeze out I did it twice everything went back to normal immediately. Topped off the coolant haven't had a problem since

yes on some models air will build up and block heater core make sure coolant is full raise front end off the ground rev engine till heat comes on shut engine off lower car van etc fill to top with coolant