The car's air conditioning system primarily conditions the air, which means whilst cooling the air in the car down, it also helps to ensure there isn't too much moisture in the car, so it stops the air getting too humid. The car's air conditioning unit also cleans the air in the car by filtering the dust and other particles from the air inside the car.
The aircon controller unit controls when the car's air conditioning system is activated.
When you press the button on the aircon, the air conditioning will not activate until the aircon controller unit tells it to. The car's main Electronic Control Unit (ECU) decides, from information provided by specialised sensors, the right time for the air conditioning to start working. The car's ECU will monitor various factors like the temperature of the car, the speed of the engine and the temperature of the engine coolant to check not only if the air conditioning is needed, but also if it is safe to start the aircon compressor which sets the whole process in motion. When all of the factors have been checked and assesed, the ECU will tell the aircon controller unit, which is set to operate the heating and air conditioning system, to start cooling the car and the process will begin.
There are various symptoms that would indicate a new aircon controller unit is needed, ranging from the aircon not working at all to it working intermittently. If the air conditioning in your car is working but airflow is in only one direction, for example, it is blowing only towards your head or feet, it is likely to be a problem with the aircon controller unit and you may need to buy a new one.