Hi. Car battery and remote control battery were changed. CL would not work using the remote,when the key is used and the door being opened the alarm goes off. Car starts up and works but the alarm goes mad! Was advised to decode the key. How do I do it? Where do I get the code from? Do numbers on the back of the remote represent anything? Civic 2005 1.6 thanks

That's because the battery was disengaged for a longer time than the computer could remember about the remote.
Usually, it just takes time for the computer to re-learn. No action on your part is usually necessary. Otherwise, the computer will have to be looked at by a competent mechanic who will re-educate the computer.

Give it time... and next time you remove a battery, do so with the least amount of
Down-time so the computer will retain its memory.