Odd A/C issue, no air from any vents, but fan is running
 

This has happened twice, both times on a hot afternoon after 1-2 hours of driving. We usually leave the AC in full auto mode and just adjust the temp as needed. After working normally for at least an hour, the system will simply stop blowing air out of all vents-dash, floor, top, and rear. Selecting anything in manual mode does nothing to change it. Increasing the fan speed makes the fan spin faster, yet almost no air is delivered from the vents. The very little delivered is cool air. Stopping and shutting off the car for a few minutes seems to reset things and it works again as normal.

Is this the pressure switch problem others have had?

Not fun when we are 40 miles from home driving through Sarasota FL.

Could the evaporator core be frosting up? Then when it's shut down for a few minutes it thaws out enough to flow air again. I had a Hyundai XG350 that did the same thing when it was very humid outside.

In addition, there is a servo motor that….
 

… open vanes to direct the flow. That me become stuck as well.

+1. For OP, try putting it into recirc. mode and see if it then keeps working on longer drives. That way it doesn't have to keep dehumidifying the outside air as it works, which increases freeze up. But, it shouldn't have that issue anyway, so have it looked at, esp. if still under factory warranty. Variously the sensor in the inside A/C panel could be whack, or your HVAC drains that deal with the condensed water may be clogged. If you park it and come back to a big puddle underneath it may be more toward the former than the latter. If no or very minor puddles when parked after using it a while on a hot + humid day, suspect drains.

Thanks for the replies. I am going to rule out any freeze-up. The system drains just fine and the very small amount of air coming from the vents is cool--the flow just drops to almost nothing. We did shut it down for 10 mins and given the heat 90+ degrees plus 75 mph on the interstate, heat from the motor, I would expect things to melt quickly. No change. Once we stopped and turned off the car for 10 mins everything worked normal again.

Still under warranty, so we shall see.

This happened to me as well on a very hot, very humid day with multiple short trip stops, non-highway driving. Very weak air flow with no cooling. Turning off the car for a few minutes and restarting cured the issue. Dealer told me under certain conditions, it's not uncommon.

Scott-
The evaporator is fairly well isolated from outside heat sources, think about how long it takes a glass with ice cubes to melt on a hot day. Ten minutes wouldn't do it.

SnagIt-
I'd be curious to hear what and why your dealer thinks it is normal. I had some problems on a road trip, after 3-4 hours in blazing heat I hit squalls and 100% humidity, and coming out of them, the AC got weak enough that I was sweating slightly--no matter how I tried to set it. Variations on the same theme during the rest of the trip. AC OK in the morning or under overcast, wimped out during high heat on long highway runs. Which sure sounds like some type of evaporator icing up to me.

IIRC that can be from low gas pressure, or from a bad orifice valve? or some other pressure regulating part internally. Its on my ToDo list to get into the shop, because I know how expensive it is to fix these things out of warranty, and how hard it can be to figure them out IN warranty.

I've been in too many old GM luxury products to accept "sometimes the AC wimps out" as being an acceptable condition. But then again, for many years they were, after all, the unchallenged world standard in cold air.

If you want to go w/ that, okay. But...

So it drains, that's good. But if partially blocked, you may not be able to see diff. from cups to quarts. Drains aside, even on a now 40 year old Audi (with essentially no insulation compared to current) I could freeze it up at highway speeds in the non-humid CA environs if something was wrong with overall system. Older designs were a long copper wire that went to the inside panel the air blows past and then transmitted the temp. thermally. Whether that old style or a sensor, if it isn't reading correctly and panel freezes up, air flow drops off more and more.

Again, simple suggestion for now: put it on recirc and see if that changes things. If it gets better, a huge pointer it is freezing up, regardless of debating exactly how.

Living in Texas where it is always hot and humid; if the AC stops working properly and Audi considers in "normal" or "not uncommon" I wouldn't be driving an Audi.

That being said, this is my second Q5, it's not uncommon for it to be over 100 degrees with 60-90% humidity and I do a lot of stop and go driving. I have never had an issue.

If I remember correctly, if your low on refrigerant (from a leak) the evaporator can run colder and ice up easier. OR more accurately, part of the evaporator gets too cold and ices up. Once that happens, air flow decreases and the rest of it ices up even worse.

It's also possible your evaporator coil is dirty, air flow is reduced to a point where its running too cold and ices up.

However, I thought modern AC system cycled on and off using a low pressure switch or thermostat on the evaporator coil, so I'm wondering if there's another issue there.