Originally Posted by mikez

I remember correctly, the key fob runs on a rolling code with a different sequence every time you lock and unlock the car.

Every time you lock the door the sequence will be XXXXXXXXX12X3XXXXXX (where X represents random digits and the 123 will be the identifier) from the car's computer, that will get programed into the fob, and it will only open if the sequence of code from the fob is the same as the the one earlier.

It is much like most garage door opener cept there will be more X digits making it near hard for most brute force method and random number generators to crack.

There is a possibility of someone reading your rolling code with a receiver, but my guess would be the sequences of X must be different, but the identifier will be the same before the car's computer will accept it. So they will still need to figure out the identifier digit and placement, but it changes every time you lock the car door.

There is also a very small chance someone else has the same sequence as you at that instant in the world, and you should be able to drive away in their car lol

Originally Posted by 650IS350

Unless they have a blank keyfob and know your factory settings for your immobilzer and reprogram that into the blank keyfob. Just like when you loose your key fob and the dealer would have to delete it from your cars memory to not accept the lost one and accept the new one.