I have recently acquired two IBM xSeries 360 servers, both the 8686 model. I tried the first one with no disk in it, simply booting with the monitor connected. When I booted, there was no video output, no POST, and the screen remained blank, although the system did boot. The only error shown on the LEDs was to check the log. The 7-segment debug display gets stuck on 5.6 (where the . is at the top of the display), and reboots after a while. I followed the various troubleshooting tests specified in the manual (removing all peripherals, removing most memory, taking out all but one processor etc), but it still would not work. I then tried a separate video card, and POST indicated that there was a monitor error (one long beep, three short beeps). No matter what I could not get any video output. After checking the log with RSA, the only thing written there was "Resetting system due to an unrecoverable error".
On the second server, the same issue occurred: no video output and the check log LED lit. No configuration of components could make the server boot properly. The log indicates no error in this one, however. It just indicates that the server powered on, then powered off after I shut it down. The debug 7-segment display on this one gets stuck on 1.3, and does not reboot.
The logs are available here for the first server: here, and here for the second server: here.