The wife's Esseesse decided a few weeks ago that the climate control was no longer going to have any effect whatsoever on the temperature of the air coming out of the dash leaving her with the heating stuck on fully hot.
I did a lot of reading about how it was a dash out job to replace the heater box incurring a £2k bill at a main dealers. I didn't fancy that at all. I'd also stumbled across a forum post by someone that'd fixed a few without taking the dash out. I figured that if someone else could do it then I'd probably be able to figure it out.
Firstly let me just add that prior to the failure it'd always had only a vague grasp on actual temperature so these pictures also covet the fix for that.
The 'stuck on one temperature' issue is that the drive tab on the temperature control flap snaps off from the flap itself, the flap isn't available as a separate part, the only way to fix is to replace the whole heater box, to replace the heater box the dash has to come out, to get the dash out the doors have to come off. Hence the £2k main dealer bill...
However,
1st remove the drivers knee airbag (I didn't actually unplug it, just swung it round out of the way) and unplug the elictric power steering to gain access to these 2 bolts



Undo them with an extension down through the steering column adjust handle slot





On the outer side of the footwell behind the obd...



With these undone you can remove this....



Which gains you access to the temperature control motor, remove these 2 bolts....



With this removed I drilled x2 1mm holes in the white plastic cog that connects the motor the to the flap and put in 2 roll pins...



And using the mark left by the snapped off tab, drilled x2 1mm holes in the corresponding positions on the end of the heater flap....



That would've been it and, to quote Haynes, refitting is the reverse of removal, except that I wanted to see if I could improve the accuracy of the climate control.
So inside the motor there's a circuit board with a potentiometer, I cleaned this and the motor with aerosol contact cleaner and regreased the gears.....



After reassembly it works as it should and temperature is more accurate.
I reckon the roll pin fix is equally applicable to the blend flap on the other side of the heater box but access is easier as you need only remove the glovebox.

Hope this helps 

Interesting they fail though, the design of the solenoid looks near-identical to most other HVAC solenoids, and they often outlive the car they're fitted to...!

Yeah, when you search the part number it crosses to a lot of different vehicles.
The design is crap on the heater box though and the motor has more travel than the flap itself which means the drive tab on the flap is under pressure and the motor itself is bogged down when the climate is set to either lo or hi. I assume that's why the S4 heater box is different,  from pictures it uses the same stepper motor but driving a couple of cogs. I wonder if that sorts the range of travel issue.
I tried adjusting the end positions on the scanner but couldn't reduce the range sufficiently.

Hi, I am going to have a go at this tomorrow, could I ask the size of drill bit & roll pins you used?
Thanks for any help.
Regards, Tim