HVAC Inconsistent When Cold in 996

I have an HVAC issue (mostly heat, it appears, but haven�t had much opportunity to test the A/C lately) with my �02 996. When the car is cold, the heating system will blow cold for about the first half-hour or so of driving (I can get all the way to the airport with only an occasional puff of heat � see below). The dealer has checked it, found a fault code for the Temp Mix Servo, and replaced it (part #996-573-919-00). It worked fine for maybe a day or so, then started acting up again. They ran the diagnostics, it coded again, and they tested the unit itself and again it seemed faulty, failing when flexed slightly, so they replaced it a second time under warranty. The first replacement was not an old unit, the date code was from August of 2010 I believe, so we were all surprised it would follow an identical failure mode. The second one worked fine going home, but then the car sat a couple days and on the way to the airport was almost totally out again. Now it is in failure mode almost all the time. I checked it with a Durametric and indeed did get a Code 26, though when I activated the flap motor, it seemed to do so (I could hear a whirring from the dash) at least once (perhaps I was just lucky that time, since it IS intermittent). All things point to the flap motor, but there�s gotta be something underlying that�s causing them to fail. Some reasoning: 1. I don�t think it�s the blower motor because when it blows, it blows fine. 2. I don�t think it�s a temp sensor, because it seems to know the interior is not warming up, as it sets the fan to HIGH speed, blowing that cold air as strong as it can! 3. It works from time to time on hard cornering and strong acceleration, kind of like an old SC with failed rear fan blower. But unlike those, it DOESN�T blow heat when in neutral and standing still, using revs via the accelerator pedal as a blower motor, only with momentum. 4. If it were charging or alternator-related, I�d expect revving in neutral would work as well as under load. Two other questions: 1. I am unable to clear the code with the Durametric software. I ran the HVAC test set, then tried to clear, and nothing. Is this just because the underlying fault is still there? It gives me a message indicating that it might not be able to �communicate with the particular module� or something like that. 2. How hard is it to get to the Flap Motor? I don�t have manuals, and the Alldata online description is kind of terse. It sounds like maybe I can get to it from inside the passenger compartment, is that true? 3. Is there anything related to the flaps themselves that I might be able to do? Can they be repaired/replaced separate from the rest of the HVAC unit? Perhaps lubrication might help them? Thanks for any help, Blaine

has anyone checked the coolant temp, level, bubbles - 30 minutes to get heat? that aint right... is the car under any warranty?

I'm wondering if you have an electrical fault somewhere in the wiring causing the mix servo to fail...when they replaced the previously "bad" units did they do any diagnosis on the servos themselves? Are they shorted?

I would try bleeding the coolant system. You may have an "air Pocket" in the heater coil. I use a Uview 5500 vacuum pump. The heater coil is a high point in the system which might collect air bubbles.

I don't think there's a problem with the actual heat - it does blow hot when I accelerate, turn hard, or, strangely enough, if I turn off the HVAC completely I seem to get a little warmth. There's also the fact that I am getting a servo code. I'm going to take it back in and will ask them about bleeding it, though. The tech did check the removed servo and said he could get it to fail with just some flexure of the casing, which is what's really perplexing - is there something that could cause the units to fail even if they were good going in? For instance, I was wondering if the flaps were binding, and perhaps the motor had enough torque to overcome that initially, but then burned out with regular usage over the course of a day or two. Or am I wandering into fantasy-land with that idea? My Alldata subscription just expired, so I will probably re-up that, but I was wondering if anybody had any ideas on how hard it is to get to the A/C unit to check my theory? I tried going in from the battery area, but realized pretty quickly that's the wrong side. Before I start tearing at interior panels, I'd like to know how they are supposed to come out 'the right way' and which are the ones to remove to get the easiest access to where the servo would be. I've got an appointment to take it in Monday for them to check it out again, so I'll update you if I get an answer from them. Thanks, Blaine

Could it be the HVAC unit itself? my audi had this issue and it ended up being the control unit.

Man, I hope that's not what it is. It is an Audi unit, or at least the controller in the dash is, I'd imagine the main unit is too. Could I ask what it ran you to get THAT swapped out? Thanks, Blaine

You're referring to the control unit in the dash, or something on the main HVAC 'box' inside the dash? Thanks, Blaine

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Well, In my 2000 A6 the heat was acting funny as you said. I would turn on sometimes and most of the time it would be cold. It was definately a solenoid/ flapper motor issue. I luckily have an Audi Allroad which is the same model as my A6 so i swapped the HVAC unit to begin troublshooting and that was the problem.

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the control unit in the dash, if you know someone else with a 996, see if you can swap the units real quick

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Symptoms are indicative of air bubble/vapor lock.

Just to give an update, this last time when I took it in, the mechanic looked at the juncture where the linkage connects to the little arm on the servo, and apparently there's a little rubber grommet there that had gotten pressed into the fitting and was allowing the linkage to bind up. He pulled it back to its original position and put a bunch of trim adhesive behind it to hold it in place, and it seems to be working fine now. I still haven't looked at where this actually is, but I think I know now how to get in there. Unfortunately, I had some foot surgery and haven't been doing much driving nor mechanical stuff lately, but I'll be driving it tomorrow after quite a bit of sitting still, so we'll see how it goes. Of course, now we're already starting to get days in the 70s and 80s here, go figure! Thanks, Blaine

Darn I just finished reading all of this and was going to say only thing else I could thing of is linkage is off. I was wrong, it was binding

Hi, sorry to bring up an old topic but seems i'm having similar issues. My air con was gassed last week, and now runs freezing cold as it should. I don't however have any heating. The coolant level is fine, no leaks, and the pipes running water in and out of the heater matrix under the hood are red hot, so hot water is getting through the matrix correctly. The blower fan works fine, and adjusting the temperature to max heat is moving the hinge (servo?) correctly under the dash. I can hear the motor moving and i also stuck my head in the footwell and watched it moving back and forth as I adjusted the temperature. I've tried changing where the air emits fom vents, feet etc but still only get non-heated air. Basically it's just blowing ambient air. Living in the UK (cold and wet weather) having a warm cabin is a must so any suggestions would be great. I keep reading about mixer flaps which allow more or less heat through from the heater matrix, but i'm not sure where to locate this flap to inspect it. Is it right up under the dash or in the compartment that holds the matrix? A photo or image would be greatfully received. Cheers for any help guys.

Sounds as if the servomotor is moving but maybe not the mixing vane/door.

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Have you found a pic for this? The HVAC in my inlaw's 2003 996 blow hot/cold respectively, but it doesn't seem to like small changes in temp setting. I.e. - If I move the HVAC from 70 to 76F, I don't necessarily feel a difference on the blower. If I move to 86F (it actually says HI for hi-temp), then it gets *really* hot. If I move to 85 or 83, it's still hot, but barely. So I am thinking that it's some sort of cabin temperature sensor or "flap" that needs lubricating or something. Can anyone make a suggestion?

Any news on this guys?