Problem

It blows hot air into the cabin, although the temperature slider is at 15 degrees celsius and it is tropically hot in the car.

Or...

...it just blows cold air, although it is so cold inside that icicles is hanging from your earlobes.

• The first thing you should try is to remove the control unit which is bolted to the wall above the gear lever and go over the soldering points. Clean with eraser and the end of the shaft (which you sharpen with a pencil sharpener) on a brush with stiff bristles. Remove debris with the brush. To solder up the eight connectors, secure them with small wooden blocks, etc., so they do not get out of position. (Between contacts you can use a suitably thick block of post-it notes: Tear away just so many sheets that it is confined between the feet and on the outside put one disassembled clothespin made of wood with rubber bands on the ends.)

• It may help to replace the sensor in the ceiling. Or you can install an "emergency" switch that is shorting the wires to the sensor. You will then get cold air from 15 to 29 degrees, and warm air on the 30th. The switch can also be nice to have if the motor that controls the hot/cold damper fails to twist the damper far enough to warm. It will then be standing clicking, even if you set the temperature at the coldest (when it is below 15 degrees in the car).
NOTE! The switch must be designed so that you do not injure yourself if you bang your head into it.

• The motor that controls the hot/cold damper may be defective.

• If you do not get hot air, it could also be that you don't have enough coolant.

The temperature control.

Suspect solder joints. Here are cracks, so two of the contacts are loose.

Attached switch on CX.