On Honeywell HC900, when the CPU status indicator flashes for 4 times then stop for a while. what does it means?

 

Assuming that you are referring to the controller status LED at the top; like most operational status LED's, its action is defined in the Installation and User Guide:

The Installation and User Guide for HC900 Hybrid Controller, document #51-52-25-107 can be downloaded here.

http://hpsweb.honeywell.com/Culture...umentation/hybrid/hc900/Manuals/documents.htm

(take out any space(s) the forum might insert into the URL)

see Table 27, page 167, pdf.