If the C0 detector is original, I would say it needs replacing as you state that it will not test successfully. As for the rest, did you hook up the batteries the same way the originals were? If you did then you are trying to run things on 6 volts instead of 12 volts as the originals were wired in parallel to maintain 12 volts and if you hooked up the new batteries the same way you only have 6 volts. You need to hook up the new batteries in series to get 12 volts. That will probably fix all your problems. Keep us posted.
