Apple Hardware Test hangs every time :0(

You can try running Memtest86. You can create a bootable Memtest86 USB drive using Etcher (Mac/Windows/Linux). Option Boot the USB drive and select the orange icon labeled "EFI".

AFAIK using "D" or "Option D" does the same thing (I just tried them the other day on a 2012 laptop).

Just because Crucial memory is being used doesn't mean the memory is Mac compatible. Macs are very picky about the memory used and these Crucial sticks could have other timings that are incompatible with the Mac (I've personally seen this many years ago). Years ago Crucial would actually label the Mac compatible memory as "For Macs" on the label on each stick, but they stopped doing this a few years ago. The only way to know whether the memory is compatible with a Mac would be to take pictures of the RAM and contact Crucial tech support to verify compatibility.

It certainly seems these memory modules are not compatible with the Mac.

I agree with @MrHoffman that you should probably return the laptop and have the seller fix the issue for you. Although will you be able to trust that everything is working Ok even if the issue is straightened out? Sometimes it takes a while for issues to be appear or to be noticed.

joeyjohnson wrote:

...

... these macs are only designed to support 8GB of RAM, not 16.

That is NOT correct, however, you must install EXACTLY the correct spec RAM NOT FASTER or it will be flaky.

I agree that I may have ended them prematurely, ... That definitely should not happen ...

BUT: you can not reliably conclude that anything specific is wrong, only that the test is hanging. There is no evidence that RAM is the cause of the hanging.

I wish I coiuld afford memtest86, but at $40-50 I just can't.

Memtest has been around for a long time, and there are many different versions posted online. Look a little harder, and you may find information for a free version that can be used to create a stand-alone bootable test for MacOS directly.

... but rather than beat your head against rock, what spec (especially SPEED) are the RAM DIMMs in the machine? If they are faster than the spec for the MacBook Pro, they will be flaky, and fail from twice a day to 10 times a week.

Apple Hardware Test runs flat out with no intervening cycles to update so much as the mouse cursor. The long test always looks like it hung.

You are not being nearly patient enough. Bad RAM gives you an error message, it generally does NOT hang the test.

Unless/until the test finishes you can only conclude that you cancelled the test, nothing more.

AHT is not likely to be bad.

The other possibility is that some of the RAM you have is perfectly good, but does not happen to work in your computers. In that case it will fail Memtest, provided you run it for a long time -- at least overnight, preferably longer.

16GB should work perfectly, PROVIDED it is not broken and is EXACTLY the correct spec for your MacBook Pro(s). The built-in memory controller in these Macs has NO ability to adjust specs in any direction. Faster (which conventional wisdom says should work fine) is flaky in practice and FAILS in these Macs.)

The 2012 laptop with updated system firmware provided by macOS 10.12.6 can access the online version of the Apple Diagnostics so there shouldn't be a problem if you hold the Option key or Option + D keys to access the online diagnostics. As @Grant mentions the Apple Diagnostics will test 16GB of memory just fine even if it is third party. It is possible the diagnostic will provide an alert that non-Apple memory is being used.

The RAM is correct. DDR3 1400, all specs checked thoroughly.

A 2012 laptop requires PC3 12,800 ( or 1,600MHz).

Keep in mind that there are a lot of other timings which are not listed on the label and even Apple's public documentation does not list the information for these other timings. This is why it is critical to make sure the memory is guaranteed to work in a Mac (or any specific computer). The only way to do this with this memory since you did not purchase it is to contact Crucial support to confirm it is Mac compatible.

I have put 2 x 8GB cards in the spare machine and run the quick test (which took a minute or two last time with 8GB Ram in). With 16GB in, it hung on the quick test at 22 seconds, just like the other machine. But this 16GB ram is DIFFERENT ram! Its Hynix (specs good also). So thats two different machines, two different 8GB RAM cards, and failures every time.

This isn't a complete test as you could have a defective memory slot on the spare machine. Sometimes a slot will work just fine with a small module, but as soon as you install a larger module you may encounter memory issues or maybe the system has a power issue since a larger stick of RAM requires a bit more power and the timings are much more critical especially if installed in pairs. You cannot just stick both sticks in and say it is bad. You need to test each 8GB module individually in each slot. It is not uncommon for the slot nearest the Bottom Case to have bad solder joints causing intermittent failures. @Grant & I have no way to confirm the Hynix memory is even good or compatible or whether the other laptop's memory slots are good.

Sometimes you can have incompatibilities between two memory modules even if both modules are good. Plus there is a possibility that even your "good" Hynix memory or even your spare laptop is now damaged. I've seen times where a system can damage a component such as memory where installing that component in another system will cause damage to that second system. Now even if you install a known good working module in the second system it could damage that module as well. I've had a few repairs over the years where this has happened. This is always something to consider when troubleshooting.

The 2012 laptops can handle 16GB of RAM as we have upgraded many of our organization's 2012 laptops without issue assuming the laptop is working correctly. As I've mentioned working with memory is not a simple task especially when you have so many unknowns here with the laptop and even the memory. This is why contributor's on these forums recommend only buying directly from Crucial or OWC. And always buyer beware when buying used items.

joeyjohnson wrote:

...

when they were made I am pretty sure it was only intended for 8GB ram.

Apple tests its feature exhaustively. But it only tests what Apple is currently selling, and only while that model is currently available. When these Macs were sold, Apple only sold 2@ 4GB max.

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Standard RAM: 4 GB Maximum RAM:16 GB*

Details: 4 GB of RAM is installed as two 2 GB modules, no slots free.

*Apple officially supports a maximum of 8 GB of RAM, but third-parties have determined that this model actually is capable of using up to 16 GB of RAM with two 8 GB memory modules.

In the US (and many other countries), site sponsor Other World Computing sells memory -- as well as other upgrades -- for this MacBook Pro.

from:

https://everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-i7-2.3-15-mid-2012-unibody-usb3-specs.html

.

AHT does NOT fail with 16GB of good RAM in a working MacBook Pro 2012.

There are no "hard limits" in those tests -- they will test all that is present and available and working. If good RAM above 8GB is present it will pass. If bad RAM above 8GB is present it will fail.

There are a few obscure older cases where you literally install 6GB, but the I/O addresses punch a hole in it and you can only "see" 5GB. In that case that 5GB passes with flying colors if it is working properly. (apple only sold those models with 4GB max.)

joeyjohnson wrote:

Yep pretty sure that's the explanation. If Memtest confirms all this ram is good then I will consider it confirmed that AHT fails with 16GB , in which case hopefully someone else will find this thread useful some time

Just because Memtest86 doesn't report a failure does not mean the memory is good. It just means that the tests used in the diagnostic were unable to detect a failure or the conditions for the failure were not met by these tests.

There are times when Memtest86 may even incorrectly report a failure when there is no actual failure due to a bug in the programming (I've seen this happen many years ago).

Diagnostics are just tools that may be useful in troubleshooting a problem.

Return it to the seller to sort it out, assuming the seller is willing to accept the return with your attempts to repair.

Attempting repairs on a newly-acquired device that arrives malfunctioning can be... perilous.