Finding Hardware Faults: Exploring AHT & Apple Diagnostics

Alongside the tools provided for identifying software issues with your Mac, Apple also makes sure that you can examine your hardware for possible faults.
These hardware diagnostic tools have evolved over the years, so in this tutorial I will cover the two incarnations that exist: the venerable Apple Hardware Test (AHT), and the newer Apple Diagnostics tool that replaced it. In this tutorial I'll show you how you can use these helpful utilities to keep an eye on the components that make your Mac tick.

The Evolution of AHT

It used to be that most of the tools required for diagnostics lived on the separate install discs that came with your Mac.
As OS X become more self-sufficient, the AHT became part of the operating system itself. When Lion introduced the idea of a Recovery Partition, AHT was one of the diagnostic utilities that made the move to your hard drive for easy access.
As of June 2013, AHT has been entirely replaced by Apple Diagnostics. Both tools accomplish the same tasks, so I want to show you how they both work.

Before You Run AHT/Apple Diagnostics

By controlling both the software and hardware distribution for its systems, Apple is able to optimize the two halves to work together seamlessly.
The result is that hardware is specifically chosen and optimized for OS X’s needs, so the chance of a problem appearing is relatively low. Since technical support is about being thorough, you’ll want to rule out more probable scenarios before resorting to hardware diagnostics.
Going through these steps first also ensures that you have a more complete picture to present to Apple’s official support team if you end up having to contact them with your hardware problem.

Step 1. Check for Software Problems

If you’ve recently installed new apps, or changed a setting in System Preferences, or otherwise tinkered with the programs on your Mac, then chances are that whatever issue you’re seeing stems from a software issue.
If you suspect this may be the case, try using Disk Utility and/or Safe Mode first to rule it out.

Step 2. Rule Out New Peripherals

Similarly, if you bought any new peripherals—printers, input devices, etc.—then try restarting your Mac with those items unplugged to see if the strange behaviour is related to an incompatibility there.

Step 3. Prepare for AHT

Assuming it is none of the above, your problem may be hardware-related and you’ll need to use AHT to help uncover the culprit. Before starting, make sure your computer is connected to a power supply (if it’s a laptop) and that you’ve disconnected any external devices you don’t need. Ideally, just the mouse, keyboard, and display should be connected.

Using Apple Hardware Test

Since it came first and is installed on a wider range of computers, the first incarnation of the hardware diagnostics tool that I’ll examine with you is the Apple Hardware Test.
If your Mac was introduced after June 2013, then you can skip ahead to Using Apple Diagnostics and follow the instructions there instead.

Step 1. Shut Down Completely

To begin, you’ll need to make sure your Mac is fully turned off—not simply asleep. Turn your computer off completely by clicking the Apple icon in the top left of your screen and selecting the Shut Down option.

Step 2. Boot Into AHT

To access AHT on a computer with a Recovery Partition (including an external one—make sure it’s connected), you’ll need to press the power button to boot your Mac and then immediately press and hold the D key on your keyboard.
Holding D
Hold D immediately after pressing the power button.
Tip: Make sure you press the D key before the startup screen appears or OS X will boot normally and you’ll have to restart and try again.
If your Mac doesn’t have a Recovery Partition, then you must first insert the appropriate installation disc for your computer (Applications Install Disc 2 for computers shipped with 10.5.5 or later, otherwise Mac OS X Install Disc 1). With the disc in the drive, restart your Mac and hold down the D key as above.
If you don’t have access to your installation discs, there is one final option available to you: AHT over the Internet. The process is identical to the above, except you use the key combination Option-D instead. This will attempt to run AHT from Apple’s servers, a process that works on most post–2010 Macs as long as they’re running the latest firmware updates.

Step 3. Pick a Language

After about a minute, AHT will be ready and will prompt you to select a language.
Select your language preference to proceed.
Use your mouse to select your preference and click the arrow to proceed, or simply use the arrow keys on your keyboard and press the Return key instead.

Step 4. Select a Testing Regime

AHT will then probe your hardware to gather information.
AHT probes your hardware before performing any tests.
Next, you’ll be prompted to choose between standard testing and a more comprehensive version. If you want a quicker test, simply click Test or press the Return key again.
Ready for Testing
You're ready to test: click Test or pick the extended testing option.
If you select the checkbox entitled perform extended testing, then you’ll invoke a much lengthier assessment that meticulously examines each sector of RAM. Both tests check for faults in the CPU, GPU, fans, firmware, sensors, and other internal components.
While the extended testing regime is more thorough and can unveil subtle problems with specific RAM blocks, it takes far longer to complete—more than an hour on a machine with lots of RAM, compared to the five or ten minutes that the standard test typically takes.

Step 5. Interpret the Results

The trickiest part of using AHT is actually doing something with its findings. Once the test has finished, you’ll be presented with the results at the bottom right of your screen under Test Results.
Testing Complete
If nothing is wrong, AHT will give you the all-clear.
If nothing is detected, you’ll simply see No trouble found. Otherwise, you’ll find yourself staring at a series of cryptic codes. The codes are designed to identify specific component faults, but they’re not very friendly to the layperson. Luckily, I can help you decode them.
The first four characters of the code indicate the component that has failed:
  • 4ETH: Ethernet controller
  • 4IRP: Main Logic board
  • 4MLB: Logic board controller
  • 4PRC: Processor
  • 4HDD: Hard disk
  • 4MHD: External disk
  • 4YDC: Video card
  • 4SNS: System sensor
  • 4MOT: Fan motor
  • 4MEM: Memory module
  • 4AIR: AirPort wireless card
If the error is with a sensor (4SNS is displayed as the beginning of the code), then you can further interpret the problem by checking the first character of the error code’s second half; I indicates an electrical current sensor, V shows a voltage sensor, and T denotes a temperature sensor.
The location of the sensor in question can be gleaned from the second character:
  • A: Ambient air sensor
  • B: Battery
  • C: Central processors (CPU)
  • D: DC (direct current)
  • e: PCI-express slot
  • F: FireWire port
  • G: Graphics processor (GPU)
  • H: Hard disk
  • h: Heat pipe (heat sink)
  • L: LCD display
  • M: Memory or memory riser boards
  • m: Misc. (i.e., battery chargers)
  • N: North bridge (motherboard controller)
  • O: Optical drives
  • P: Power bus
  • p: Power supply
  • s: Palm rests for laptops
  • W: Airport Wi-Fi card
Realistically, you don’t need to do any decoding of the error code you receive. If there’s a hardware fault, in most cases there will be very little that you can do yourself, so simply call Apple Support with the code and follow their advice.

Using Apple Diagnostics

If you thought that using AHT was cumbersome and difficult, then you’ll be happy to find out that Apple agreed with you; that’s why they upgraded the tool, renaming it Apple Diagnostics, and made sure that it’s much more user friendly.

Step 1. Shut Down Completely

To begin, you’ll need to make sure your Mac is fully turned off—not simply asleep. Turn your computer off completely by clicking the Apple icon in the top left of your screen and selecting the Shut Down option.

Step 2. Boot Into Apple Diagnostics

To access Apple Diagnostics, press the power button to boot your Mac and then immediately press and hold the D key on your keyboard.
Tip: If you have moved your Recovery Partition to an external drive, make sure it is connected before you follow the steps.
Connect to the Internet
Connect to the Internet to allow Apple Diagnostics to offer suggestions.

Step 3. Wait for Testing to Complete

Unlike AHT, Apple Diagnostics does not offer any options—there’s only one version of the test and it takes about five minutes to complete.
Testing in Progress
Sit back and relax while Apple Diagnostics verifies your hardware.

Step 4. Interpret the Results

Unlike the hideous codes offered by AHT, Apple Diagnostics attempts to make the information more useful to normal users.
Once your test has completed, you’ll either see a simple No issues found message, or you’ll see a screen with the problems listed. Each problem is described in simple terms like There may be an issue with the onboard memory, and alongside the message you’ll see a code that you can give to Apple Support that will help them identify the issue.
Test Results
Much more readable results are offered by Apple Diagnostics.
Important: Don’t forget to make a note of these error codes before proceeding any further!
To help you handle the issue, Apple Diagnostics can take you into Recovery Mode if you click Get started; this will restart your computer and bring up Safari to direct you to the appropriate Support options.
Support Options
Follow the directions to get support for your hardware issues.

Using Resets to Fix Common Faults

If your error code indicates an issue with the logic board, processor, sensors, or an Ethernet controller, then you may find relief by resetting the Non-Volatile RAM (NVRAM), also called Parameter RAM (PRAM) on old Macs.
This area of memory is not cleared when the system is shut down, and OS X uses it to store low-level settings for things like system volume, resolution, startup disk, and more.
As a last resort, you can also reset the System Memory Controller (SMC), which is responsible for managing the power supply, ambient light sensor, thermal sensors, sudden motion sensors, and other similar functionality.

Step 1. Reset NVRAM/PRAM

If it becomes corrupted, NVRAM can cause all sorts of strange errors, so try resetting it by rebooting your Mac and holding down Option-P-R before the startup screen.
Keep holding the keys as your Mac reboots and only let go once you hear the chime a second time. Note that you may need to adjust your screen resolution and other settings after an NVRAM reset.

Step 2. Reset SMC

On Macbooks with removable batteries, disconnect the power supply, remove the battery, then press and hold the power button for five seconds. The SMC reset has been performed and you can re-insert your battery.
On newer Macbooks without removable batteries, shut down the computer and plug the power supply in. Press the Shift-Control-Option keys on the left side of the keyboard at the same time as the power button. Release all keys and the power button at the same time, then restart your computer normally.
For desktop Macs including the Mac Pro, Mac Mini, and iMac, SMC resets are performed by shutting the computer down, unplugging it for fifteen seconds, and then plugging it back in and starting up normally.


Performing diagnostics on your Mac is a great way to better understand how it works, and while you don’t need to include these tools in your normal maintenance, knowing how to use AHT or Apple Diagnostics is essential to getting the support you need when something goes wrong.


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