I had a similar problem that many have posted about. Duovalve sticking causing Hot air on drivers side vent while A/C comes out other vents.

It is interesting that when the duovalve goes it seems always to be on the drivers side, no one complains of the reverse, the passanger blowing hot with the others working well.

I just wonder why this happens when both of the valves (in the duo) are identical, why does the drivers side one go? Is it heat related since it is closer to the engine. Just bugs me?

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1991 300D 172,000 (totaled)
1998 E300 Turbodiesel 82,500