Chosen Solution

Hello, I purchased OWC RAM of 4GB, 2400 MHz and tried in my iMac. It works fine. However I cannot use all 4 slots at a time, iMac doesn’t boot. Plz help I tried these:

  1. Both new RAM (4GB+4GB)-Boots fine
  2. Both Old RAM(4GB+4GB)-Boots fine
  3. One old & one new-Boots fine I confirmed that all 4 slots work fine. All 4 RAMs work fine It is just that I cannot add more than 2 RAM at a time The following combinations too, work fine. 1,3 slot-Boots fine 1,4 slot-Boots fine 2,3 slot-Boots fine 2,4 slot-Boots fine Configuration is 3.8Ghz intel core i5, 8GB 2400MHz 18,3 Thanks Rajesh Update (07/11/2018)

Added RAM Pics Update (07/11/2018)

Back side is here

Take a snapshot of the RAM labels so we can read them Somethings not right here. Adding images to an existing question The needed modules are: 260-pin PC4-19200 (2400 MHz) DDR4 SO-DIMM’s. Which is what you stated you had. All I can think of it maybe mis-marked.