Hello. The proper procedure before running any diagnostics is to clear any existing hardware logs. The Error Code 2900:0221 represents a previously registered error and diagnostic test failed because the system picked this historical error (2900:xxxx). Consider updating the BIOS and iDRAC firmware. Otherwise, all should be fine for now.
One of our R815 servers (fitted with quad AMD 6380 Opteron CPUs) halted and powered off during the night last week and on arrival in the server room, I noticed the LCD display had turned orange with a warning about a CPU voltage regulator being out of limits. After disconnecting the AC power cords, waiting a few seconds and reconnecting I powered the server back on again and the LCD message cleared and the server booted normally. So I ran the UEFI hardware diagnostics and it came up with one error:  Test Results: fail Device : IPMI test : IPMI System_Event_Log_Check Error Code 2900:0221 Msg: IPMI - Nov 04 18:08:34 2015: processor:: Voltage sensor (1.2V VDDR) state asserted "CPU1"  Is this something to be worried about since the server has been running at full load on all 64 CPU cores for 5 days without any problems? Or does the error message actually refer to the IPMI/BMC processor CPU and not one of the Opteron CPUs? Andy 
R815 Error code 2900:0221 CPU voltage sensor