How to use Apple's hardware diagnostics to find and fix Mac glitches

When a Mac starts to act up, sometimes the problem is more than just a rogue software update. Sometimes, a hardware component goes bad. Here's how to diagnose and fix simple hardware problems.
Written by David Gewirtz, Senior Contributing Editor

Image: CNET

I have a late 2012 Mac mini. Well, actually, I have four of them. But one of them was sick. In this article, I'll show you how I figured out what was wrong and then fixed it.

It started a few months ago, when I installed Security Update 2016-001 for El Capitan. Immediately after running the update, my machine crashed over and over again. I did an in-place OS restore in order to preserve all my files and settings. That resulted in a crash to a black screen, right in the middle of the restore. I ran the restore again, which seemed to succeed. The machine returned to reasonably reliable behavior.

For a while.

After a few weeks, I noticed that a development tool I run regularly wasn't behaving itself. It would launch and properly display the splash screen. But every time I touched the menu bar, it crashed.

This problem was repeatable. The application was unusable. I rebooted. I had the same problem. I did another in-place OS restore. I still had the same problem.

My first assumption was that the weird behavior was caused by an update to that program. But I checked, and it was actually the same version of the program that I had been running for the past year. The developer had upgraded the program, but hadn't offered any kind of upgrade discount. Since it had been meeting my needs, and running perfectly, I had chosen not to buy the upgrade.

However, once the crashing started to occur, I downloaded a trial version of the upgrade. I had the same problem. It (and only it) crashed when touching the menu bar. Now, I had verified that the behavior was repeatable on multiple versions of the same program.

I contacted the developer, and we went over the crash logs. There appeared to be an error generated in the main UI code for the Mac, but not for that application.

A few days later, another application suddenly wouldn't launch. Or, more precisely, it launched, then immediately exited back to the desktop. This error wasn't as reliable, because it stopped once I rebooted. It was still odd.

To diagnose the problem, I ran Disk Utility. My first go-to troubleshooting action is usually to repair disk permissions, since that's long been an easy way to fix minor problems. Gack! That's right, El Cap removed the disk permissions repair option because it interferes with system protection. Special.

Even so, I did run the First Aid operation on my SSD. Everything turned out to be fine. Except my applications were still acting wonky.

At that point, I started to wonder if I had a rare Mac hardware problem. I've had a lot of Macs over the years. I have had one or two hardware problems, but they've always manifested right after purchase. I've never had a machine fail years into use, so hardware failure was pretty much at the bottom of the list in terms of things I considered checking.

The generation of Mac minis prior to the currently shipping one were quite user modifiable. Mine is a generation older than the newest ones. When I bought it in 2012, I added my own RAM sticks, and swapped out the dog-slow included hard drive for an SSD.

Since I had run out of ideas as to why the machine was malfunctioning, I decided it couldn't hurt to run the hardware diagnostics.

It's important to understand that Macs released before June 2013 were shipped with the Apple Hardware Test (AHT), while those produced after June 2013 come with Apple Diagnostics. Since my machine is a late 2012 machine, I used AHT.

How to run Apple's diagnostics

  1. To initiate the Apple Hardware Test or Apple Diagnostics (depending on what year your machine was built), shut down your machine completely.
  2. Make sure your machine is completely powered off.
  3. Press the Power button and immediately press and hold the D key on your keyboard.
  4. If you boot into the Finder, power off and repeat.
  5. You will launch into either Apple Hardware Test or Apple Diagnostics
  6. Select your language, if asked
  7. Run the tests and write down the result
  8. Google the result and look for appropriate answers if the diagnostics report hardware failure.

Once in the diagnostic system, I got the following screen:


Apple Hardware Test running a scan

Once the test completed, you can see that I did, indeed, have a hardware error.


That there's your problem right there. Yer 4MEM/9 is out of whack. Tough break, that.

Typing 0x7bda4190 into Google yielded nothing. But typing 4MEM/9 into Google immediately yielded a discussion board post that implied one of my RAM sticks was bad.

My RAM was four year old aftermarket Crucial RAM, which had been powered on and running on the machine pretty much nonstop for all that time. Replacing all 16GB was about a $70 expense.

The next day, I had the RAM in hand. My late 2012 Mac mini is extremely easy to open. You simply flip it over, turn the big black base slightly, and pop it off. The RAM is right there. RAM Replacement takes about five minutes. The only trick is making sure you press in the new RAM sticks at about a 45 degree angle.

Once that was done, I put the little machine back together, and plugged all its peripherals back in. I powered it up, and tested my software. Everything that had failed was now working.

I've been running with it for about a week. I haven't had any additional problems, so I'm pretty sure that minor hardware issue was the problem. I'm very glad it didn't turn out to be some sort of out-of-warranty motherboard issue, because that would have been a lot more expensive.

I was also glad I don't have the latest model of the Mac mini, because RAM isn't field replaceable in those machines. In this machine, the $70 memory fix was quick and easy. I'm glad to be back up and running.

If you have weird glitches, remember to run either AHT or Apple Diagnostics. It could answer questions you might otherwise tear your hair out over for months.

You can follow my day-to-day project updates on social media. Be sure to follow me on Twitter at @DavidGewirtz, on Facebook at Facebook.com/DavidGewirtz, on Instagram at Instagram.com/DavidGewirtz, and on YouTube at YouTube.com/DavidGewirtzTV.

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