Hello, Give a try and install only 2 memory moduls (16GB) first, use the slots, where your original modules were. I just bought 16GB (2x8) Crucial memory (CT2C8G3S186DM) to my iMac 27" Retina (Late 2015) and I mixed it with 8GB Apple RAM, they work fine together. Unless your modules fault, they should work in your iMac. Regards, Attila
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