Stupid HVAC prb has everyone stumped

Not sure if anyone has this prb. Basically, every now and then (probably 30% of the time I start the car), the whole HVAC sys doesn't work, so all the lights are inoperative in those '4 circles' in the dash (save for the rear defrost). If it's on or off, it stays that way until the next time I turn on the car, i.e. if the HVAC is off, it won't suddenly work when I'm using the car, and vice versa.

The dealer has changed so many parts and I thought that they finally solved it, but no, the prb still exists!

Parts replaced:
-blower resistor motor
-heater control
-control unit

I'm not a trained tech, but how hard can it be to diagnose this prb??? Any ideas?

this happened to me the 2nd day I had the Z4 and I took it to the dealer and he "reset" the car. This was done by disconnecting the batt cables and touching them together - problem solved and never a problem since in 6000 miles...Any dealer knows this trick....you lose nothing but have to reset your clock.

Originally Posted by Glen E

this happened to me the 2nd day I had the Z4 and I took it to the dealer and he "reset" the car. This was done by disconnecting the batt cables and touching them together - problem solved and never a problem since in 6000 miles...Any dealer knows this trick....you lose nothing but have to reset your clock.

Thx for the post. I really hope that it's that simple, will let the dealer know.

Dealer doesn't think it's a good idea - afraid that touching the batt cables will short something...I dunno. In any event, still have this unsolved prb. Wondering if anyone has more input.

For those that did the 'master reset' aka 'touching battery cables together', how long have you been prb-free? Any side effects? Thx.

Dealer is not too swift - this is what some of the BMW service reps do...."short something out"??? all you are doing is draining the residual energy in the capacitors....

Ok, guys, I may have stumbled upon a possible solution. You know how presumably everyone starts the car by pressing the brake, and then pressing the start/stop button? I've been doing it the other way around, i.e. holding the start/stop button and pressing the brake to start the car. So far, no more HVAC prbs (knock on wood). Pls note that if you do it the 'normal' way and the HVAC malfunctions, you can't try to restart it using my way afterwards as the HVAC needs time to recover (as per usual whenever it malfunctions).

I'll leave it up to you guys to field-test this to see if my solution actually does work. Just call it the 'Tranquility Fix' if this does work.

Update: I have tried my solution (i.e. hold 'start button' then press brake to start the car...instead of the usual method of other way around) for several months now and it definitely works. Call it the 'T-Fix' from now on and remember where you found the solution.

This problem seems to be sort of random. I had the problem, took it to bmw and they replaced the electronics.

A few weeks later i saw the problem again, and then never again.

My best guess is that moisture shorts something out in the electronics. every time i've had this problem the entire front window fogged up right after starting the car.


Just next time this happens to you, check if it's early in the morning when you start the car.

I don't think that it's weather-dependent...it has something to do w/ the electronics/circuitry or else I would have faulted even doing the 'T-Fix' sometime or other.

Originally Posted by JavierBayArea

This problem seems to be sort of random. I had the problem, took it to bmw and they replaced the electronics.

A few weeks later i saw the problem again, and then never again.

My best guess is that moisture shorts something out in the electronics. every time i've had this problem the entire front window fogged up right after starting the car.


Just next time this happens to you, check if it's early in the morning when you start the car.

Actually from everything i have read on this it seems software related. the issue is that these AC units are more like our radios and PCs now they have to properly be booted in order to be used. I suspect that the T-Fix above is energizing the brain before the button is pushed which is allowing the AC to initialize without any issue. Of course this is merely speculation but sounds like a logical reason based on what everyone posts on this issue. Unfortunately I havent had the issue to test the theory.

NOTE: i am not implying that the AC is a PC I am comparing the vehicle brain to a PC because that is where the initialization of the AC begnis.

Yup, it may be software-related. When I perform the T-Fix, I notice that sometimes the backlights for the '4 circles' delays for a few seconds before switching on.

Hey all ...

Well, I'm the one that did the battery-cable reset technique to fix MY electrical issues -- which happen to be the audio not working. Standard symptoms that people see with this as the HVAC issue ... when it happens when you start the car, it stays that way until you stop the car and let it sit for 30 seconds, etc. The battery-cable reset works for me for about a month or so before the problem happens again, then I reset it again and good again for about another month or so.

Well, after about a month, the audio stopped working again (right on schedule). SO, I tried tranquility's "T-Fix" and it worked! Basically, after starting my car normally, the audio didn't work. In the past, I had no choice but to wait until I stopped the car for about 30 minutes or more and then when I started it, the audio would work. However, this time. I just killed the engine, started it using the "T-Fix" (press the start button, hold for about one second, THEN foot on brake to start the car), and to my surprise, the car started with the audio working again! No way that would have cured itself so quickly in the past.

So, I think we have a viable short-term workaround. This IN ADDITION to the battery cable reset trick should help people out to minimize these issues ... but your mileage may vary.

/LesC