memory problem with iMac late 2015 I wanted to upgrade the memory on an iMac (Retina 5K, 27-inch, Late 2015), which had 2x4GB chips. I bought 4x4GB chips (I only needed 2, but bought so many to use in other machines).
None of the 4 chips worked - as in, swapping them out one by one, making sure they were well seated, the 2 original chips worked fine, but with the 4 new ones, the machine would not boot at all.
The original chips, made by &quot;SK hynix&quot;, have a sticker that says:

4GB 1Rx8 PC3L - 14900S - 13 - 13 - B4
HMT451S6BFR8A - RD NA AA  1638

The new ones, made by &quot;Komputer Bay&quot;, have a sticker that says:

4GB PC3-14900 DDR3
2RX8 NON ECC 1.35V

The seller (on Amazon) states these new chips are made for this model of iMac. Is the seller wrong? Or could I have been given 4 defective chips?
Here is the link to the Amazon page, which states &quot;..for Apple iMac 275K (Late 2015)&quot;. Thanks to @monomeeth for that - it is possible I was sent the incorrect type.
Thanks!
Your model iMac uses 204-pin PC3L-14900 (1867 MHz) DDR3 SO-DIMM memory chips, so you need to ensure that any chips you use meet this criteria. 

You can go for faster memory (so long as everything else matches), but note the Mac won't use it at those faster speeds. I mention it just in case you can't find compatible chips at the correct speeds. Also, there's no mention of voltage in the official specs, but the 1.35v is fine. 

As an aside, your iMac can actually use up to 64GB RAM (not just the 32GB officially supported by Apple).