Fixed my Duovalve
 

I had the classic "left vent hot" symptom. That side of the duovalve was sticking open. I saw a pretty good DIY at benzworld I think. But it did not really have an explanation for the problem or solution. But following that general procedure, here is what I did:

Pulled off the electrical connector
Removed the 5 T10 torx screws
Carefully lifted off the cover which contains the electrical coils
1 at a time carefully removed a plunger and spring making sure not to lose the spring!
Cleaned a dark coating off of the plunger with my finger nail and a paper towel, being careful not to scratch anything.
Lubed the thin part of the plunger with a little silicon faucet grease and reinstalled along with the spring of course
Repeated for second plunger
Carefully reinstalled cover
Reconnected connector

Thanks! This happened to my son's 2000 C230K and didn't learn until later that I probably could have fixed it myself. Indie has no time for this and just put in a new one.