Hi the key coding is in the pcb inside the key, the transponder is just the medium between the pcb and the pick up ring around the ignition barrel or key recognition point printed on the dash to the right of the climate control, no one could buy just a transponder and start your if they don't have the code thats embedded in the pcb, you might have to re pair the key but there are plenty of videos on youtube showing how to resurrect the communication, once the ECU recognises the code it will work.
Steve.