This is relatively easy to fix.
The dual side climate control system uses small sensors mounted behind the control wheels. The sensors are badly soldered and fall off the board inside the unit.
Remove the whole unit. Open it up and you need to find the small, black sensor that sits near the edge of the rotating knob. This will either be attached, but not connected to the electrical tracks, or like mine, rattling about in the case.
Needs a soldering iron to reattach. I then added a blob of hot melt glue gun glue to support it.
I wrote a how to with pictures at the time, but I can't find it now.