I have a 2001 A6 4.2 and I'm having an issue with the A/C. The passenger's vent blows cold the passenger side center vent blows cold, but the driver's side center vent blows warm and the driver's vent blows almost hot air. It's as it gets warmer from right to left.

I thought the air came from the same place. Any ideas? It's not much fun sweating while driving and the passenger is too cold.

I'd remove the cabin filter. With the A/C on, fan onhigh, spray silicone
down the filter hole. Set the temperature for heat and then A/C.

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2001 A4 1.8T Q Casablanca White, Tip, ESP, Xenon and Premium

When I turn the A/C on the vent plates (for lack of a better term) close off the outside air. I sprayed silicone with the fan on high, the ECON switched off, and the heat on HI (because the vent plates were open) then switched it to LO but the air is still warm. Did I do what you explained correctly? How much silicone should be used?

Do you have dual climate control?

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Yes, it does have dual climate control.

The ducting is probably disconnected. Try to look under the dash to see if anything obvious is wrong. Also, your local library may have a repair manual you can check out to see if you may learn something new about the a/c system. I know my local library has a repair manual for my B5. Good Luck!

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Find someone with a VAG-COM to diagnose the problem.
It might be an interior sensor.

http://vag-tool.kicks-ass.net/

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2001 A4 1.8T Q Casablanca White, Tip, ESP, Xenon and Premium

If it's not an interior sensor, I have run into this exact problem and despite my disbelief, it was due to a slightly low refridgerant level. I didn't think that could cause it, but the shop foreman told me that part of the evaporator will be warmer and cause the air on one side to be warmer. I don't know if thats accurate, but I do know that recovering and recharging the system fixed the problem. If no faults are present, give it a try.

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Seth Guenther