First things to check are your freon level and your duovalve. Both are common culprits for this -- run a quick search and you'll find my recent posts (and other more helpful posts) on exactly the same problem.

I took my duovalve apart and found all to be well. I then added a little freon and the problem was resolved. Pays to try a few other things before you go in for major repairs!

Cheers, John

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'98 E300TD (W210) | Sold 6/09 with 205k and counting
'04 VW Touareg V10TDI