The replacement keys only work if you have provided Kryptonite with the correct key code number. If you registered the keys with the wrong key number, the replacement keys will not work. You could send your lock back to Kryptonite to have it re-keyed, but the cost would probably be close to that of a new lock.

Kryptonite does have a program that would enable you to use one key for multiple locks. The only flaw with that is that if a crook somehow gets one of your keys, he can take all of your bikes. Still, for folks who use four or five locks, a single key is tempting.