Late 2011 model, A1278 / 2.4 GHz i5 or 2.8 GHz i7 processor.

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Lower SO-DIMM RAM slot not working

When both slots are populated, the system only records 2GB of memory being available. I have tested both RAM slots by booting with just one slot populated. The laptop will boot with RAM inserted in top slot alone, but not bottom slot alone.

Is there an easy way to replace these slots or diagnose the problem?

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Tell us about the RAM you are trying to use.

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The RAM that came with the MAC. Both chips are working, as both will boot from the top slot.

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@Louis Rossmann gave the following answer (which has subsequently been deleted) which I actually found useful (my solution is to max out top slot)! I'll place it as a comment and remove if it is found factually inaccurate.

Bottom slot is soldered using BGA soldering, top slot is soldered using through hole soldering. It is common for the bottom slot to die.

The pads slowly come away from the board or disconnect from internal vias, usually from a drop or from someone soldering near the slots with the RAM still attached. There is no fixing this, just max out the top slot and call it a day, and thank Apple for using BGA for soldering a RAM slot to the motherboard.

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So I bought this i.e. Crucial 8GB DDR3 1333 MT/s CL9 SODIMM 204 Pin 1.35V/1.5V Memory Module for Mac, and it's done the trick. The laptop registers as having 8GB of RAM and runs like a beast once more.

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James Owers will be eternally grateful.
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