If for some reason your car forgets your key's RF id then it will need to be recoded. Below are the steps to recode the key to the car. I know this works with the new key, I'm not sure if it works on the old key as well. Have all the keys with you when recoding! Old Key/New Key information.
Step 1. Get in the car and shut all the doors.
Step 2. Put key #1 in the ignition and turn it to Accessory and within 5 seconds, turn the ignition to Off and remove the key.
Step 3. Hold the key up so that it is in direct line of sight of the receiving antenna in the rear windshield. Press and hold the Unlock (Arrow) button and press and release the Lock button three times within 10 seconds. Release the Unlock button and the car should lock and then unlock the doors to let you know you did it right. Repeat step 3 for any other keys within 30 seconds.
Step 4. Insert a key into the ignition and turn it to Accessory. This terminates the key coding procedure.
If it doesn't work try it again or try starting with another key.
The order in which the keys are initialized is important. If they are not initialized in the same order as they were originally initialized the key memory functions will not be assigned to the correct key. Gosh, that's stupid.
Don't like my instructions then you can read instructions posted on the net by other people.
http://forum.e46fanatics.com/showthread.php?t=592731