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

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

2016 12-inch MacBook in Rose Gold color

Causes of Mac Hardware Failures

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 a Mac. The reason the Mac is stuck could be a hardware or software problem. Running the Apple Hardware Test can narrow down the cause.

Apple hardware fails from time to time, with the most common failure being RAM. 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 operating system version installed on the Mac.

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

For all 2013 and newer Macs, Apple changed the hardware testing system to use 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. It was also included on a USB flash drive 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 the Mac's startup drive. If the Mac has Lion or later installed, you're 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 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. It's the USB flash drive included with the purchase.

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

If you purchased an Intel-based Mac in or before the summer of 2008, the AHT is found on the Mac OS X Install Disc 1 DVD included with the purchase.

AHT on PowerPC-Based Macs

For older Macs, such as iBooks, Power Macs, and PowerBooks, the AHT is on a separate CD 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 the 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 the Mac and immediately press and hold the D key. Continue to hold the D key until you see a small icon of a Mac on the display. When you see the icon, release the D key.

  5. Select a language from the list of languages that can be used to run the AHT, then click the right-facing arrow at the bottom. The Apple Hardware Test checks to see what hardware is installed in the Mac. Wait for the hardware probe to complete. When it's complete, the Test button is highlighted.

  6. Check what hardware the test found by clicking Hardware Profile. Look through the list of components to ensure that the Mac's major components show up correctly. If the configuration information appears to be correct, select Test.

    If something seems wrong, verify the Mac's configuration by checking the Apple support site for the specifications on the Mac. If the configuration information doesn't match, the device may have failed and 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 finds issues with RAM or graphics. If you suspect such a problem, start with the shorter, standard test.

  8. Click Test. The AHT displays a status bar and error messages that may result from the testing. The test can take a while. You may hear the Mac's fans rev up and down. This fan activity 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. If you see an error code in the test results, look at the error code section below for a list of common error codes and what each means.

  10. If everything seems okay, you may still want to run the extended test, which is better at finding memory and graphics problems. To run the extended test, select the Perform Extended Testing check box, then click Test. The extended testing takes more time than the standard test.

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

Apple Hardware Test Error Codes

The error codes generated by the Apple Hardware Test tend to be cryptic 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

Additional Troubleshooting Tactics

Most of the error codes indicate a failure of the related component and may require having a technician look at the Mac to determine the cause and cost for a repair. Before going to an Apple Store or sending your Mac off to a shop, reset the PRAM and reset 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, repair it using Disk Utility, which is included with OS X, or a third-party app such as DiskWarrior or Techtool Pro.

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

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

If you misplaced the optical media or USB flash drive, you have two choices. You can either take the Mac to the nearest Apple Store or call Apple and order a replacement disk set.

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

The other option is to take the Mac to an Apple Store. The Apple technicians can run the AHT for you and diagnose any problems the Mac has.

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