Honda Radios have a protection code. They hold this in a volatile RAM i.e. it needs power to retain this information and it loses the code when changing the battery. This article was written in 2007 and is probably pretty worthless now. Jan 2019
Responding to the Code prompt.
Less driven people will need to find the code and use the preset number buttons to enter the code. The following sites will help. The Honda Civic Type-R owners site who have a thread called Radio Code Help, although you may need to login in to read it. I also found Edmund’s Car Space helpful.
See also google:honda radio code
Obviously not the only people to suffer from this problem.
The code is issued on a white card, which is often located in the glove compartment or best kept with the car’s documentation and manuals.
Edit 26 Jun 20007 A friend, i.e. Mike Ramchand claims to have traded his car in order to avoid buying a new code, although it was a Peugot, not a Honda.