I bought a 5K iMac 4 months ago, with the stock 8gb of ram. I bought an extra 2x4GB off Amazon (Crucial Ram). But then, it looks like I cannot use all 4 ram slots at the same time. The iMac won't even start, only show a black screen. The Crucial ram sticks are 100% working, since I tried removing the original ram sticks, using only the new ones, and the mac boots and runs fine Right now, I am running the iMac with 3 of the 4 sticks, so at the moment I have 12gb of ram. I tried using the top 3 slots, it works. Bottom 3, it works. 2 slots works too. But as soon as I insert a 4th ram stick, it stops working I have resetted the SMC and NVRAM. Still no luck Also updated Mojave to 10.14.2 (latest version)

Any ideas? I would like to see if there's anything I can try before setting an appointment with Apple for this, since it's my main working machine, and I need to work on some projects right now.

Thanks a lot!

Here's the Ram kit I have bought:

Crucial 8GB Kit (4GBx2) DDR4 2400 MT/S (PC4-19200) SR x8 SODIMM 260-Pin Memory

I've read multiple comments on Amazon about this kit working fine with 5k iMacs...so I'm not sure what's going on here. Thanks

  • What are the specs of the RAM you purchased?
    – Allan
    Jan 21, 2019 at 23:58
  • I have added more details about the RAM I have bought. Thanks
    – larin555
    Jan 22, 2019 at 12:18

3 Answers 3


Not all RAM is created equal. The way motherboards handle things is radically different. For example where an ASUS PC motherboard might take mismatched RAM, the iMac will not. To ensure compatibility use RAM that has the same:

  • Clock Speed
  • Voltage Requirement
  • Latency timings

It's best to buy identical RAM sticks. They should be the same manufacturer and the same model number. Occasionally even RAM with the same model number can have different modules on the chip. (This is very rare )

Can you update your question with details about the Manufacturer, Model number, Clock speed, voltage Requirement, and latency timings of your RAM?

UPDATE: According to the Apple website, the RAM you bought should be compatible: https://support.apple.com/en-us/HT201191

Might be time for the genius bar.


If, as you say, each of the sticks works individually, but not altogether, then I would suspect the logic board to be at fault.

Also, from personal experience today: I got a black screen on my 2018 Mini, because I hadn't seated the new sticks correctly. The new Macs with T2 chips don't make memory error beeps at startup.

  • That's what I read on the web, but I am always pressing hard on each sticks to make sure they are correctly inserted. So I highly doubt this would be the issue. Is there any software available to test if the ram slots are working fine?
    – larin555
    Jan 23, 2019 at 14:41
  • Yes, D for Diagnostics at Boot. support.apple.com/en-gb/HT202731
    – benwiggy
    Jan 23, 2019 at 16:33

It still might be that the RAM isn't seated correctly, even if you pushed hard. Upgrading my older Mac, I got the exact RAM specified by Apple as being compatible.

Following the directions on the Apple support site, I felt each RAM chip click. But nothing afterwards.

Finally in a fit of frustration, I hammered all of them in with the back of the screwdriver handle and it worked perfectly. Percussive maintenance isn't usually recommended for Macs but in this case it worked.

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