I was having the same problem with my '94 C280. Hot air out of the side vents and cool air out of the center vents. My fix was replacing the "duovalve" or dual valve. If you go to any website that sales parts and look it up you will see a photo. It is located under the cover near the firewall on the passenger side of the engine bay. A fairly easy do-it-yourself project.

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• 2001 E320