Hi
you are correct. The air-con control unit will not allow the pump to be activated if it detects that the ambient temperature is so low. It sounds like you have damaged the outside temp sensor when you collided with the curb, it is afterall in quite a vunerable place.
Take a look at the underside of the left corner at the front of the vehicle and check for damage and possibly any loose wiring hanging down.
Rich.
It should resemble this....
It's good news that the plug is still on the end of the wiring but chances are the sensor could have been knocked off and is still in the road. It is usually clipped to the outside of the under trim where your wire is hanging down and looks like this....
If it's not there then they are available from your dealer's parts dept and arent very expensive.
BMW Technician
5 years main dealer experience.