hvac: cooling front, heating rear
 

Well today is not my lucky day. First a big chip in the windshield and now on the way back from picking up family from the airport we noticed that the A/C is cooling in front as configured but the rear center and sides are blowing hot air.

Has anyone had this before? I suspect one of the servo motors in the air tubes might be stuck.

Do you have four seat control or just two. If 4, obviously depends on rear settings.

Otherwise, reason #87 why to get VCDS for the now $200 and the scan the module to see what it says. On my D3 W12, almost 10 years in the rear temp setting control started acting up. Would go up, but not down unless I overrode from MMI in front. VCDS scan. Module error. A week later, cheapest on on eBay showed up (Germany actually). About $100. Out and in 20 min, function restored. Hope it might be easy like that, plus value of scan shown. And yes, could be one of the servos/position sensors. Then you have to go diving into trim/upholstery typically. Fortunately not very common, even when older. VCDS will identify any that are acting up, by sensor label (like G123) and sort of a description of it.

If under warranty still, always get dealers to fix these. And scan whole car for hiccups like these so even if you don't know they are acting up (like no one in back seat...), then you do and it goes on pre warranty expire punch list. First one above that you have can pay for cable. The warranty pre expire full scan ("lab work") can save you 5x its cost or more at dealer rates and parts prices. BTDT.

MP4, does the scan actually flag codes that are out of spec or do you need to cross reference on what the codes should come back at? I'm thinking the later could take a long time and would make it easy to miss something. I have 2 months left on my warranty before my 2 year CPO kicks in and I would like to find as many issues as possible before the CPO kicks in. Considering VCDS but not certain I know enough to use it properly...

i know when my kids are in the rear seat and monkeying with the HVAC system is gets wonky and I have to fix it from the front.

When you run it, you can scan all modules. It quickly tells you which ones have any issues/codes--shown in red. Then processes for 10 minutes to open each one and extract the codes. To short cut I often just abort and use the list to open those showing in red. Then it gives you pretty specific information. There are thousands of specific codes, but on something like this it might say periodic short or outright defective module, or failed to communicate. Or, it might say, issue with flap V113. You need to then look up flap so and so, but usually there are some words to orient you--a lot better than the "P code" generic scanners the are typically engine only just for starters (tiny fraction of possible warranty issues) and don't provide much info beyond a code and big picture area of motor or smog system. Because of that text, even if you aren't super knowledgable, it can give you enough to use with dealer service. Like I had a code on my D3 somewhere in wiper fluid system. Turned out it was a valve IIRC at one headlight. There was enough to the text that I just headed in with a general hand wave about headlight squirters working weird. I knew their typical first step was a similar scan, so that would trigger them to do so. Hundreds of dollars saved, and one of easily a dozen like that over the years where issue was fixable but subtle. Said another way, they typically don't self initiate repairs unless related to something they are already doing or a recall, so you need to report it as an issue and whatever you observe for behavior. Then they document as a "customer concern" for the service techs to look into. If you know there is a code but can't describe behavior, at some point you can just say, hey my friend scanned it and said there is a code in the HVAC module and thought I should bring it in. Or, I saw a code and heard about this TSB so bringing it in.

BTW, don't erase any codes beyond just dumb stuff like key battery low. And, take it in for last warranty hurrah weeks or a month before it clocks out. Thus, time to order parts, do repairs, have you assess, maybe scan again, etc.

At a more technical level to some of your questions, yes it gives actual performance parameters too. Called measuring blocks. Thus it might tell you the heater flap that is supposed to open 90 degrees is only reporting 32. In a scenario like that, there was likely a code saying flap so and so was not working correctly. You need to know more about program, what the specs are against the measured value and so on. Used for things like fuel flow issues or things where there is a sensor and some hard to track down issues--again the code may say "lean" but not get you enough data yet to isolate. 95% of mine over many years show at code level and either directly (at least half the time) or by searching posts against the code (most of other half) I can pretty much isolate where the problem is at a part/repair level. Finally, on some modules it can run a self test too. Like it can cycle stereo speakers tonally, or click the motor mount solenoids, etc. It wouldn't tell you if a speaker is physically torn (though you might hear something), nor if mount is cracked open but still functioning electrically.