Use the Apple Hardware Test (AHT) to Find Problems

The AHT can usually be found on one of your Mac's install DVDs

The Apple Hardware Test (AHT) is a comprehensive application that can help diagnose hardware-related problems you may be having with your older Mac.

Information in this article applies to Macs manufactured in 2012 or earlier.

2016 12-inch MacBook in Rose Gold color
Courtesy of Apple

Some Mac issues, such as those involving startup problems, can be caused by software or hardware issues. A good example is getting stuck at the blue screen or gray screen when you start up your Mac. The reason you're stuck could be a hardware or software problem; running the Apple Hardware Test can narrow down the cause.

The AHT can diagnose issues with your Mac's display, graphics, processor, memory, logic board, sensors, and storage.

Apple hardware does fail from time to time, with the most common failure being RAM. Fortunately, for most Macs, RAM is easy to replace, and running the Apple Hardware Test to confirm a RAM failure is a simple task.

Apple Hardware Test Availability

Not all Macs run AHT. Of those that do, the method differs depending on the version of the operating system the Mac has.

  • Macs manufactured in 2012 or earlier and running OS X Mountain Lion (10.8.4) or later have the Apple Hardware Test built into the Mac.
  • Macs made in 2012 or earlier and running OS X Mountain Lion (10.8.3) or earlier use the system software disc or flash drive that came with the Mac.
  • Macs manufactured in 2013 or later aren't compatible with AHT.

For all 2013 and newer Macs, Apple changed the hardware testing system to use a system called Apple Diagnostics, which is built into the Mac.

AHT on Macs That Shipped With OS X Lion or Later

OS X Lion was released in the summer of 2011. Lion marked the change from distributing OS software on physical media (DVDs) to providing the software as a download. Before OS X Lion, the Apple Hardware Test was provided on one of the install DVDs that were included with the Mac or on a USB flash drive that was provided for the early version of the MacBook Air, which didn't have an optical media slot.

With OS X Lion and later, for any Mac manufactured before 2013, the AHT is included in a hidden partition on a Mac's startup drive. If you're using Lion or later, you're all set to run the Apple Hardware Test.

AHT on Macs That Shipped With OS X Leopard to OS X Snow Leopard

OS X Leopard (10.5) was released in September 2008. For Macs that were sold with OS X 10.5.5 and later versions of Leopard or with any version of OS X Snow Leopard (10.6), the AHT is located on the Application Install Disc 2 DVD that was included with the Mac.

MacBook Air owners who purchased their Macs during this time frame can find the AHT on the MacBook Air Reinstall Drive, a USB flash drive that was included with purchase.

AHT on Intel-Based Macs With OS X Leopard 10.5.4 or Earlier

If you purchased your Intel-based Mac in or before the summer of 2008, you can find the AHT on the Mac OS X Install Disc 1 DVD that was included with your purchase.

AHT on PowerPC-Based Macs

For older Macs, such as iBooks, Power Macs, and PowerBooks, the AHT is on a separate CD that was included with the Mac.

How to Run the Apple Hardware Test

Now that you know where the AHT is located, you can start the Apple Hardware Test.

  1. Insert the appropriate DVD or USB flash drive with AHT on it into your Mac. This step is unnecessary for Macs with Lion or later, where AHT is on a partition on the hard drive.

  2. Shut down the Mac.

  3. If you're testing a Mac portable, connect it to an AC power source. Do not run the test from the Mac's battery.

  4. Press the power button to start your Mac and immediately hold down the D key. Continue to hold the D key until you see a small icon of a Mac on the display. Once you see the icon, you can release the D key.

  5. Select a language from the list of languages that can be used to run the AHT appears and then click the right-facing arrow at the bottom.

    The Apple Hardware Test checks to see what hardware is installed in your Mac. Wait for the hardware probe to complete. When it's complete, the Test button is highlighted.

  6. Before you press the Test button, check what hardware the test found by clicking on Hardware Profile. Look through the list of components to ensure that your Mac's major components show up correctly. If the configuration information appears to be correct, proceed to the test.

    If something appears to be wrong, verify your Mac's configuration by checking Apple's support site for the specifications on the Mac you are using. If the configuration information doesn't match up, you may have a failed device that needs to be repaired or replaced.

  7. Click Hardware Test. The AHT supports two types of tests: a standard test and an extended test. The extended test is a good way to find issues with RAM or graphics, but even if you suspect such a problem, it's better to start with the shorter, standard test.

  8. Click the Test button. The AHT starts, displaying a status bar and any error messages that may result from the testing. The test can take a while. You may hear your Mac's fans rev up and down; this is normal during the testing process.

  9. When the test is finished, the status bar disappears, and the Test Results area of the window displays either a "No trouble found" message or a list of problems found. If you see an error code in the test results, take a look at the error code section for a list of common error codes and what they mean.

  10. If everything seems OK, you may still want to run the extended test, which is better at finding memory and graphics problems. To run the extended test, place a check mark next to Perform Extended Testing and click the Test button. The extended testing takes considerably more time than the standard test.

  11. Quit AHT by clicking either the Restart or Shut Down button.

Apple Hardware Test Error Codes

The error codes generated by the Apple Hardware Test tend to be cryptic at best and are meant for Apple service technicians. Many of the error codes are well known, however, and this list may be helpful:

Error Code Description
4AIR AirPort wireless card
4ETH Ethernet
4HDD Hard disk (includes SSD)
4IRP Logic board
4MEM Memory module (RAM)
4MHD External disk
4MLB Logic board controller
4MOT Fans
4PRC Processor
4SNS Failed sensor
4YDC Video/Graphics card
Apple Hardware Test Error Codes

Additional Troubleshooting Tactics

Most of the error codes indicate a failure of the related component and may require having a technician look at your Mac to determine the cause and cost for a repair. Before going to an Apple Store or sending your Mac off to a shop, try resetting the PRAM and resetting the SMC. This can be helpful for some errors, including logic board and fan problems.

You can perform additional troubleshooting for memory (RAM), hard disk, and external disk problems. In the case of a drive, whether internal or external, try to repair it using Disk Utility, which is included with OS X, or a third-party app such as DiskWarrior or Techtool Pro.

If your Mac has user-serviceable RAM modules, try cleaning and reseating the RAM. Remove the RAM, use a pencil eraser to clean the RAM modules' contacts, and then reinstall the RAM. Then, rerun the Apple Hardware Test using the extended testing option. If you still have memory issues, you may need to replace the RAM.

What to Do If You Can't Find the AHT Disk or USB Flash Drive

It's not uncommon for the optical media or USB flash drive to become misplaced over time. You won't notice that they're missing until you need them.

If you find yourself in this situation, you have two choices: Take your Mac to the nearest Apple Store or give Apple a call and order a replacement disk set.

Before you call, you need the Mac's serial number, which you can find in the Apple menu under About This Mac. When you have the serial number, you can call Apple support or use the online support system to initiate a request for replacement media.

The other option is to take your Mac to an Apple Store. Technicians there should be able to run the AHT for you and diagnose any problems you're having.