You don't say what kind of control panel you're trying to add this to, but I would guess a Vista 20 based on the symptoms. You cannot use a 4208U for this panel...what you need is a 4219, but even that may not work because Honeywell has changed the components without changing the part numbers, making the new expanders incompatible with older controls. You may need to upgrade to a Vista 20P.
Answers & Comments
You don't say what kind of control panel you're trying to add this to, but I would guess a Vista 20 based on the symptoms. You cannot use a 4208U for this panel...what you need is a 4219, but even that may not work because Honeywell has changed the components without changing the part numbers, making the new expanders incompatible with older controls. You may need to upgrade to a Vista 20P.