Hello MikelGT;

It could be that your problem stems from not wiring AI/AO modules that have been projected in the hardware configuration, or the problem could come from many different  causes. It can be difficult to troubleshoot your system with so little information you can provide in a post such as this.

But you can always ask the CPU what it was complaining about when it set the SF LED. The PLC's diagnostic buffer logs all events of interest to the operating system, including faults and their likely smcause. Open your diagnostic buffer, use the Save As button you will find in the buffer screen to generate a text file you can attach here if you would like some support troubleshooting your SF (system fault).

If you would like to keep your CPU running even when that particular fault happens, check the diagnostic buffer entries for a string such as "Missing OB, unavailable or disabled", followed by OBxxx.  Create a block of type OB, with the requested number, and dowload it to the PLC. That will allow the program to run, even in the case of a fault,  even though the SF LED will still remain set..

Hope this helps, 

Daniel Chartier