I got a late 2013 iMac i5 3.4Ghz quad core. It came standard with 2 x 4GB PC3 DDR3L 12800-1600. I bought 2x Mushkin 8GB PC3 DDR3L 12800-1600. When I install the new ram set, my iMac doesn't boot. No startup chime, no sound, no screen. Like it wasn't getting any power. I've confirmed that power isn't the issue. I removed the ram upgrades, and the iMac booted without issue (with the original ram set). So at first I thought the new ram modules I bought were bad. But because I didn't hear the 3 chimes, I decided to test them out. So I pulled the original ram modules that came with the iMac, and replaced them with the upgrades. And low and behold, the iMac starts up without issue. So now I've confirmed that all ram modules are not defective, and are of the same specs. The only difference is 2 are 8GB, and the other 2 are 4GB. This model maxes out at 32GB. Can anyone explain, or point me in the right direction to find out why this is happening, and what I can do to fix? I don't understand why each set will work alone, but not together. Thanks.