On MOST cars (not ours of course) there are 3 ways of programming the transponders. Depends on the make. Most cars can only have up to 8 keys in the memory. Notice the xfire uses the same key codes (3501-6700) as the Mercedes c-class, yes there are codes available for the x-fire but it does us no good. C CLASS - C220, 3501-6700 Dealer Key Service Key HU64P High Security Key. Crypto 1997-2009 All 83 C230, C280, C43 Transponder, Ordered by VIN

Generally you can have 2 working keys and do a series of turning on key removing and either turning on the other good key or pressing brake pedal/gas pedal or something. That lets you add/program your own new key. Here is the reference EVERYONE uses to identify make/model/year/brand of transponders with instructions. The on board programming instructions start on page 150. This reference also shows the correct key blanks for non transponder keys also.

https://www.dropbox.com/s/p1gjxusjnk...rence.pdf?dl=0

You can also use a program to "open" the computer and allow you to add in the new key.

The third is use a auto clone tool. What it does is reads the signal coming from the working key and programs the new key's signal to match the working key so the car thinks it is the old working key. It works on most cars and is the easiest way if you cant use on board programming. I can buy most transponder keys under $10

The American Key Supply Guide to Cloning : American Key Supply, The Locksmith's Source for Keys & Tools