The battery plays a role when the factory radio has an anti-theft feature requiring a code be entered. The owners manual packet should contain a small card with a 5 digit number. when the battery is disconnected this code needs to be entered. The radio will display CoDE and you'll have 5 tries before it is locked.
Disconnect the battery negative
terminal for a minute or so, make sure you leave a door open so the dome light drains any residual charge. Reconnect the battery negative terminal (always disconnect negative first and reconnect negative last). When your head unit displays CoDE enter the 5 digit code on your card. (Note that some of us Honda owners had two different code cards and only one worked).
You can always contact your local Honda dealership and ask them to fix it (for a fee).
If you don't have your codes or can't find your cards - try this:
Originally Posted by WikiAnswers
Make sure your radio is OFF. Hold down the preset buttons 1 and 6 at the same time. Turn the radio on and your SERIAL NUMBER will appear. 4 numbers first and then 4 more. Write them down (obviously) then call (800) 999-1009. Make sure you have your VIN number handy because they will ask for it. They will ask for your name and approx. milage, and your radio's serial number. They will then give you your radio unlock code--WRITE IT DOWN!
IF you have the VIN#, and are registered with Honda, you can go to their website https://radio-navicode.honda.com/
. You will need to enter your VIN#, registered phone number and zip code as well as your radio/nav serial number (retrieval process detailed above). The site walks you through the inputs and if successful, then spits either your radio code or nav code.