Then why do spare keys need to be "programmed" to work?

 

you want to reprogram your new key. put the two back to back and hold both the lock and unlock botton on the good fob and it will reprogram it. i do about 15 to 20 keys a month for everything from BMW to suburu. also it uses the magnet as an indicator in the module. kind of like the cards they give you in hotels that dont have a strip its just a low grade magnet. or a college dorm card key. maybe some of the inner city schools or state offices.

i learned these trick from a booster well i was in presion and i tried it on my wifes 2008 dingo. it actually works. then i double checked on my 90 carmaro which actually have seceraty. also worked. and it also worked on my buddys S4 and his moms A4 both of which are 2009s the megnet power changes slightly from model to model but it should be right in the range of 45 to 55 depending on the key size and make. i know my dingo is like 48 and my volvo is 52

 

I also do a bunch of keys for Dodges and Fords and it's not the key that's programmed. It's the module in the vehicle. That's why there's a maximum number of keys that can be associated with any particular vehicle. Putting the keys back to back and pressing buttons on the key fob does nothing. The keys need to be inserted in the ignition so the module can read them and store them in memory.

 

That trick is for 80's - early 90's GM Immobilizer.