This seems to be a recurring problem with the 2003 model.

Try changing the grounding cable. It worked for me when I had a similar problem. Either that or there are a few other things that have been suggested:

- Ignition relay, switch, barrell
- Alternator (or just the bushes)
- Alternator pulley to belt drive
- Multiplex bus
- Regulator
- Fuses (the alternator fuse is, I believe, 85A and can sometimes crack and be faulty without this being obvious)

Honda said they thought it was my "non-Honda" bluetooth unit that was causing the problem and that they would have to take the radio out so it would cost $286AUD. They said they weren't sure if that would fix it so the next thing to do would be to change the ECU and that would cost $2000AUD.

Very glad I came onto the forums and found out a lot before giving the go ahead to Honda.