E420 Mono Valve
 

Can a bad mono valve cause hot air to come out the driver's side vent and cold air out the center vent? This only happened for a few minutes then cold air started coming out.

afailing monovalve shows strange symtoms.best to replace.
mak

Does anyone know if this is a part that normally fails on this model? It looks like it would be easy enough to replace, but I just want to make sure it is the culprit.

I would say no. The monovalve either opens or closes. It doesn't care where the heat goes once it is open. I never had mine fail for the time that I owned it (around 70k miles? maybe more...can't remember).
I did however, have one fail on a 123 (300D). The rubber starts to tear, and hot fluid seeps past when it isn't supposed to.
btw...just in case.... You can just replace the insert, not the whole assembly.

Good luck!

Thanks, Larry. It only happened once and I remember reading a post here about hot air coming out only the driver's side vent; but maybe that was on a 140 chassis where I think there is rear a/c.