Mother board, memory or ? I was using a Asus Motherboard M4A7BLT-M LE (AMD). It according to the manual can use two memory sticks of 4 GB each. It has two DDR3 slots. I was running a single 2 GB stick in slot DIMM-A1 and everything was working. Any way I decided to upgrade the memory to 4 GB total so I purchased 2 by 2GB sticks DDR3 1600 at a kit from Kingston KHX1600C9D3B1K2/4G . Plugged them in and powered on the computer. NOTHING happened. The fans started but no beeps, no video. Pulled the "B stick out from slot DIMM-B1 and tried again. This time it worked. Swapped the sticks and it still worked with a stick in slot DIMM-A1 The memory sticks I brought were specifically indicated by the same parts numbers in the mother board handbook. Kingston KHX1600C9D3B1K2/4G. So on this basis I would have assumed that they would work. Brought another set of memory sticks Patriot PG234G1600ELKA tried again and got the same result. Could only get the computer to start with one stick in slot DIMM-A1 I had a friend check both sets on memory in his computer. Bot sets worked fine with no problem starting his computer in any combination. Thinking I had a mother board problem I purchased a ASUS H81M-PLUS (Intel) mother board. Low an behold I have the same problem. I can not get any of the sets on Memory sticks to allow the computer to start when I have a 2 GB stick in each of the two Memory slots DIMM-A1 and DIMM-B1 But if I remove the memory from Slot B the computer will start and boot normally. When I have the computer running with 1 stick only and using the ASUS H81M-Plusmotherboard I have noticed that the BIOS recognises the single RAM stick as 2 GB DDR3 but at 1333 not the 1600. I have not made any changes to the BIOS settings of either motherboard, everything appears to be on Auto. The power supply checks out ok and has a capacity much greater than needed. Indeed I have purchased a Shaw Evo 975 power supply and tried that with no effect. There is talk of a "Memok" switch on what I think is only some ASUS motherboards. can not see such a switch on either of the motherboards I have. Looked at trying to post a message to support at ASUS but after indicating the mother board model there was so much information to be filled in it was a joke. Especially as the "must have" fields should have been filled in by the motherboard specification any way. I gave up. Could not find the serial number on either of the motherboards to start with. A bit difficult to pull the machine apart while you are using it (be it with on memory stick). I have checked he seating of the sticks. there is no foreign mater in the slots and no damage in slot two on either motherboard and for it to happen on two different mother boards one would have to say is unlikely. The mother boards were brought months apart. Any ideas please.