Originally posted by wizofoz Brooksy, the flashing F___ means that the camera cannot read the aperature information from the lens. It literally cannot 'see' the F stop.
The sigma should provide this information, so there is something wrong.
Either the lens is not mated properly to the camera or the camera or lens contacts are faulty. Have you tried a different lens on the camera? If so and the F showed up properly then the fault is with the lens, if the fault stayed with the camera ( the flashing F stys regardless of the lens) then the camera needs replacing.
Either way, re-mount the lens, and if it is still flashing F___ then a trip back to the camera shop is in order.
I think before suggesting that anybody send the camera back or having it serviced, you should suggest the part in the manual about Allow Aperture Ring in the menus. Page 294. That, or making sure that the Aperture ring is set to the A position.