A broken ram module is pretty annoying in itself but trying to find it by trial and error is even worse. This simple command shows the locations of the active dimms in the system. Run it and find the missing one, in this case:
CPU#0Channel#0_DIMM#1
[root@machine ~]# cat /sys/devices/system/edac/mc/mc*/csrow*/ch*_dimm_label CPU#0Channel#0_DIMM#0 CPU#0Channel#1_DIMM#0 CPU#0Channel#2_DIMM#0 CPU#0Channel#1_DIMM#1 CPU#0Channel#2_DIMM#1 CPU#1Channel#0_DIMM#0 CPU#1Channel#1_DIMM#0 CPU#1Channel#2_DIMM#0 CPU#1Channel#0_DIMM#1 CPU#1Channel#1_DIMM#1 CPU#1Channel#2_DIMM#1 [root@machine ~]# Or:
[root@machine ~]# cat /sys/devices/system/edac/mc/mc*/dimm*/dimm_label