Are some of the pins in a memory module used only when workign in dual/triple/quad channel mode?` Can a memory module only fail when working in dual channel mode? I have an iMac (27-inch, Late 2012) that crashes shortly after logging in. I've reset the NVRAM and SMC, started off an external hard drive with Yosemite. Started in safe mode, etc. and the problem continues. It has four 8GB sticks of third-party (non-Apple). When I remove one of the four RAM modules (the fourth one if you count the slot one to four), the computer works fine. So either the module is faulty or the motherboard's memory slot is faulty (I.e.: the motherborad itself is faulty). So, to determine wether the memory or the slot is faulty, I insert move one of the three remaining healty modules to the free slot (the fourth slot). The computer works fine, no crashes. So it seems clear that the module I removed is faulty. Not so: I replace the module in slot 4 with the suspect module (the one that I removed first and stopped the system from crashing): No crashes, the computer works fine. Moving this module to slot 3, still produces no crashes, the system works fine. I go further and replace the fours modules in their original slots and the computer crashes again as expected (I cleaned the memory modules contacts with iso alcohol and blew compressed air into the ram slots, just in case). Then I remove the stick in slot 1. The system still crashes. Please: answer only to what I asked, please don't refer me to irrelevant or basic articles. Thank you.