Heater only blowing cold air

I have a heater problem on my [phone removed] C4 and have search for similar problems in the forum without finding the exact same problems. Trying to start a new post: My heater sometimes only blow cold air and when selecting Defrost (all air on front window) it only blows cold air into coup�. Sometimes when the car gets warm I can restart and get the Defrost to work and blow warm air on front window and after that I can change between Auto and Defrost without problems. Sometimes but rarely all works perfect from beginning with good heat. I have noticed small flakes of foam coming out of the vent, but not much. I have looked at the servo on passenger side (where you have your feet) which I think is moving the flap to get air to front window and that doesn't move when I can't get defrost on front window but it is moving when everything works. Can the problem be: - foam on flaps need to be repaired? - thermostat problem? - HVAC electric problem? - servo problem? Hope to get some guidance here! Many thanks Nic

Probably the foam issue but also could be a clogged heater core. They r not hard to change or even flush. Its is installed from the frunk behind the battery. I guess it could also be a t-stat stuck open but i would start by getting a visual on the vents

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If its because of the foam then its a relatively simple DIY if you don't mind cutting one piece of plastic. The flap with the foam can be accessed from under the windshield cowl, there is a plastic piece you must cut to take the flap out. The two hardest parts are cutting the plastic and then taking off the flap from under the dash. The plastic is hard to cut because its a tight space, with a Dremel tool it might be easier, I used a saw blade by hand and wiggled it back and forth. The flap from underneath is just awkward because you have to get in upside down but it isn't hard to access. My heat blows real hot now.

It is strange that sometimes it works right and sometimes not. A good way to check if it is the flaps and foam is to block the fresh air intake filter. Remove the pollen filter, cover it with a plastic bag or saran wrap, reinstall and see how things work. That essentially blocks the fresh air intake (cold air from outside). Put your HVAC on recirc (just moving around the cabin air) and see how it works.

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Nic, Try these steps... Go for a drive in the car and get it warmed up to the normal 180˚F temp setting. Once the car is warmed up, follow these simple steps and report back: 1. Turn the temperature setting on the AC control unit to " HI " by pressing the + button ( GREEN arrow) until it's shown. 2. Next, turn the fan speed ( YELLOW arrow) to the midpoint or higher 3. Last, engage the recirculate air button ( RED arrow) This should produce heat in the cabin in spite of the blend door problem with these cars.

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Excellent work around!

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I'm curious -- will this produce as much heat as fixing the problem? I've already gone in through the center stack on my car and sealed the holes with metal tape. But my curiosity is raised since both doors have holes in them. I was unaware that you could do this until AFTER I fixed my car!

I am absolutely amazed. My posts mean nothing. There is no problem. The SOLUTION is pushing the right buttons on the HVAC controls. Absolutely amazing. I am truly floored. Seriously considering not posting anything anymore, no helpful hints, no suggestions, no insight. Why bother?

Thanks for all comments and tips. I haven't had time to go to the car and try it out yet but hopefully I will this weekend and I will respond back after that!! Many thanks! Nic

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Both of my 996 cars had the unfortunate blend door problem which of course affects the heat output. My workaround (noted above) worked great for me on both cars during the winter. Of course, solving the problem would mean cutting open the blower enclosure through the center console and patching the holes in the blend doors. However, the workaround did the job to produce lots of heat in the cabin and so I didn't bother with cutting open the blower.

Snowy and cold in Stockholm today so excellent conditions to test the heater and defrost. Tried out the tip from "b3freak" today but no luck for me. Still only cold air coming out. Drove for about 20 minutes, made a pit stop for food and when I started up again the heater worked perfectly with lots of warm air coming out and possible to get air flow on front window. I will try out the tip from "DBJoe996" to see if that help. My concerns: Why is only cold air coming out? (thermostat / temp sensor in cabin?) Why can't I get the cold air to flow on front window? (servo problem?) Nic

Hmmm, there is a cabin (inside) temperature sensor on the passenger side of the dash. Could be out of whack. Also, make sure the ambient air temperature sensor in the front bumper hasn't gotten knocked out of place and is now behind the front bumper. There is a little holder it resides in in the front bumper. Next, it could be a stuck thermostat...so how does your temperature gauge on the dash read? Another source of cosmic problems is the electrical portion of the ignition switch, but in this case I don't think it applies. Overall, how does the HVAC system run? Fan and temp variable on the display? Could be the HVAC controls are out of whack as well. One last thing, manually check your coolant level.

^^^ I agree with Joe's suggestion. I can't help but have a flashback to a guy we were helping with the same problem here on Rennlist. He also tried the workaround but didn't help the situation with no heat output. Obviously something else was the culprit. Come to find out, his heater core was malfunctioning from a blockage. Pulled the old one out and replaced with another OEM, flushed the system, and it fixed the problem.

Could be heat control flap ( also know as blend door ) motor on it's way out .Not hard to check -remove foam panel under dash in passenger foot well .Motor is left of fan and will activate with temp changes with ignition on .

Today tried the tip covering the pollen filter with saran plastic wrap and HVAC on recirc but still no heat and the air can't be directed to front window. Also topped up coolant fluid which was a few cm too low. Unfortunately no luck! Noticed the outside temp sensor in front bumber hanging loose but seems connected, will open up and have a closer look. Is there a way to see if inside cabin temp sensor works? The HVAC seems to work well when I get the heat, I can change and adjust as normal so it seems ok. I opened up and looked at the heat control flap motor (what I called servo earlier) located at passenger foot well and that is not moving when it heat/defrost doesn't work, but when heat/defrost works it is moving when and I change to Defrost. Does that motor control the air flow going up on front window or controlling heat coming into the cabin? // Nic

It almost sounds like your heater core is clogged. Remove right (passenger) side cowling next to the battery in the frunk. The intake and exit heater core hoses are right there. Run car up to temp and put your hands on the heater core hoses. If one is hot and the other cold, the core is clogged.

I will check the heater core tomorrow. After reading a lot about, and trying to understand how heater works with the heat control flaps I more and more lean towards there somewhere is the problem, mostly based on that the heat sometimes work just fine which is strange. I guess the cabin temp sensor should steer the heat control flaps open and close? But when HVAC is on Defrost the temp sensor should be bypassed forcing system in full heat correct? So maybe not cabin temp sensor. Leading me to electric problems in HVAC or the motor/servo for heat control flaps. Still can't understand why when Defrost I don't even get cold air to front window or is that a direct flow not possible if heat control flap is closed? I'll get back with further conclusions :-) // Nic

The servo you were looking at controls the heat blend door .The heater (small radiator ) is always on -the blend door directs airflow around the heater and into the cabin .There are very few moving parts in this system so shouldn't be too hard to figure out what is wrong .The coolant hoses going to the heater behind the battery are marked in and out .They should be around the same temp -with the car warmed up if the outgoing hose is cold heater is plugged .Since you have intermittent heat -highly doubtful .There is another servo for the defrost flap ..Since this is intermittent you might want to check the cabin temperature sensor in the passenger side air vent it could be dirty ,loose connection or broke .II don't know what reference material you have but a porsche parts diagram might help you visualize how the system functions .

Today I ran the car until it was warm and I checked the IN and OUT hoses from the heater core and they where kind of equally warm, not burning hot but warm. After that I pulled out the cabin temp sensor , disconnected and cleaned as good as I could. Mounted all back and started up the car and the heat is on! Have used the car twice today in snowy Stockholm and it seems to be working just fine so I hope it continues that way now! Thanks to everyone for tips!!! // Nic

thanks for the update...........stay warm

Okay, problem back again!! This morning when I started the car and went for a drive only cold air again. Drove the car warm for 30 minutes and parked - after approx 30 minutes I started again and now the heat came back and Defrost and all options on HVAC worked again. So strange behaviour. Any clues? // Nic

Hate to say it but a lot of weird electrical gremlins are a result of the electrical part of the ignition switch. Sometimes it works and sometimes it wreaks havoc with various components. Might consider replacing it. There are some good tips, hints and videos on exactly how to do it. Search.

This might sound like a dumb question but when it produces heat is it really hot with the temperature setting at max ? Before I fixed my blend door I would get lukewarm heat after running at highway speeds for 20 minutes .That was in 40 f weather ...

DBJoe996: you mean changing ignition switch or the HVAC unit?

dcdrechsel: when it is working it is good heat in the system!

If the blend door isn't functioning or in the wrong position - no heat assuming the fan is working .Logical test would be to see where the blend door lever is at with heat and without .I would check at max heat temp setting .It would seem that there is a temperature sensitive connection somewhere in the circuit .One simple step could be to reseat the connectors on the climate control unit .

more .....the hvac system is all Audi components - the air handler had a Audi tag and part # -point being there may some local to you shops with Audi hvac test equipment .

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The electrical part of the ignition switch is what I mean.

Several times you've mentioned that after the car has been warmed up and then restarted, the heat is OK. Is this always the case? When the heat is working will it redirect between the windshield and the floor? It seems like something gets stuck when the car is cold and is held in place until you shut off the warm car for a moment. When you checked the heater hoses, was the heater working at the time? Can you recheck the hoses when the heater is working to see if the temperature is any different?

Hi Yes, when I have driven for 20 minutes and car is warm and I shut it off for a while it almost always works after that. I will check hoses temp when heat has been running in the car for a while to see if any difference in heat. I'll get back!