Why does iMac boot up with just one memory in any slot?

First lets check the memory - Is all of the modules the same brand and size? What is the specs of the memory? Tell us what you find.

Are you having issue in one given slot or you can put the one module in either slot just can't put both.

Test 1 OWC memory OWC1333DDR3S8GB iMac works with single stick in any slot. Doesn't work with 2 sticks in any combination.

______________________________Combinations with 2 sticks:

_______________________________________________________ _ 1. memory in front slots = reboots after apple logo appears.

_____________________ _ 2. left side back slot and right side front slot = shuts down at apple logo.

__________ 3. memory in back slots = reboots after apple logo appears.

__________ _____ ____ _ _ 4. left side front and back slots = same as above

____________ _________ ______ ______ 5. right side front and back slots = apple logo, loading then black screen for a second back to white screen and reboots

__________________________________________ 6. left bay Hynix memory right bay OWC = nothing. fans try to spin then stop and repeat very silently.

Test 2 Hynix 4GB 2Rx8 PC3 10600s-9-10-F2 iMac works with single memory in any slot.

--Combinations with 2 stick:____1. left bay slots = reboot after apple logo appears.

____2. right bay slots = same as above.

____3. back slots = boot sound no white screen then POP and system reboots and pop repeats.

____4. front slots = same pop

_____5. front left slot Hynix, front right slot OWC = reboot at apple logo then pop

______6. front left slot Hynix, front right slot Corsair = reboot at apple logo, no pop

Corsair CMSA8GX3M2A1066C7 iMac works with memory in any slot

Sumsung 2GB 2Rx8 PC3 8500s-07-10-F2 single memory works in any slots.

with 2 boot sound then pop. Pop sounds like GFI outlets pop.

I do not understand their commenting system. I typed everything so that it's easy to understand but their system bunched everything together. That's why I inserted the lines so that it will be less confusing when you read. I tried to learn that stupid !@# formatting and used some * and # but nothing happened. The programmer just doesn't get paid enough to make commenting easy for us. Anyway I think logic board is messed up. OWC RAM are brand new. What do you think?

Just use the answer field the comment field doesn't have full formatting - Sorry ;-{