The calculation is done locally, in the browser, without sending the data to a server.
The precode must be composed of a letter followed by 3 numbers (example : A123).
The precode is written on the back of the car radio, but can be recovered without removing it by simultaneously pressing keys 1 and 6 (or 1 and 5), then while holding them, press the power button.
The code calculation does not work for precodes starting with A0.