Please refer to this video: https://www.youtube.com/watch?v=BPuw6GrWX9Y&ab_channel=1AAuto%3ARepairTips%26SecretsOnlyMechanicsKnow
You can jump to 6:34 to this issue directly.
My 2014 E350 has this issue. According to this video, it is a common issue for W212 E350. I am wondering if any of you have this issue? Has any one did DIY fix of this issue? Thanks.
Senior Leigh, thanks for your reply. The worst thing of this issue is unpredictable. When you are on road for a long trip, the valve may fail. After you come back, it works fine. I have a mechanic use MB star to diagnose it. It always works fine in shop, but it could happen on the road. It happened three times on the road in last two years. I think I need to have this fixed.
I just check my 2012 E350, the warm air blown out when I switch the temp to the HI. The only difference is I turn the AC off and set two zones temp to HI. If you have an MB Star, that's pretty easy to identify the issue of the part. Maybe a reactive process can solve the problem.