If you purchased your original AB-404 6 years ago then you would have the first version of that module. The first AB-404 could be expanded but not into another AB-404. If you review the manual for that product (here http://www.channelvision.com/wp-content/uploads/Instruction-Manuals/AB-404-ins.pdf?4d6cc9 ) the AB-404 was only designed to expand into a P-1014. The newest version of the AB-404 fixed this issue, but you cannot use the new version with the old version for expansion. As far as the keypads, to troubleshoot, you would want to remove one of the original keypads on the original AB-404, the one that is working, and test each of the keypads having an issue in that known good zone output. You do not need to use AB-134 keypads, those are just the newest version of the same type of amplified ABUS keypad. For further troubleshooting you would need to contact Channel Vision technical support directly at (800)840-0288.