How to run and interpret Apple’s Hardware Tests on your Mac

CrashReporterPrefsIconXIn order for OS X or any software to run properly, your Mac’s hardware must first be in running order. While symptoms of crashes, graphical artifacts, or other corruption can be indicators of hardware malfunction, one way to help determine if hardware problems are the root of the issue at hand is to run a hardware test.

When you purchase your Mac from Apple, the system includes hardware tests in a couple of locations. The first is on your boot drive, but in addition if your system came with a gray restore DVD then tests specific to your system will be available on it as well. For those without DVD drives, and those created after 2010, Apple’s Internet Recovery services can be used to download and run applicable hardware tests.

Running the Tests

To invoke the hardware tests, insert the restore DVD that came with your system. If your system is new enough that it did not come with a DVD drive, then skip this step. Now perform the following actions:

  1. Reboot your Mac
  2. Hold the “D” key down immediately when you hear the boot chimes

If your system loads the tests, then for older systems you will see a small image of a computer and a processor chip appear on your screen, followed by an option to choose your language. If this does not appear and the system boots normally, then the tests were not properly loaded. If your Mac supports Internet Recovery, then in step 2 above, you can hold Option-D to force the system to load these tools from the internet, where it will download them from Apple’s servers.

When you have chosen a language, then the hardware tests will load. For older systems you may see a tabbed window on a blue background, with a big button labeled “Test” in the middle of it. Below this button is a checkbox for enabling the extended memory tests, which will scan your installed RAM in great depth. This option may take hours to complete, but is a recommended step to take anytime you have purchased a new system (even brand new from Apple), or have installed new RAM into your system.

When you click the Test button, a progress bar will appear and show you the time remaining for the tests. During the course of the tests, you may hear the system’s fans spin up and down, but this is a normal occurrence, provided the test interface is still responsive (ie, the mouse still moves).

Apple Hardware Test running

Apple’s latest hardware test versions show a gray scroll bar (click for larger view).

For more modern systems such as the Retina MacBook Pro, you will see a gray progress bar appear that says “Checking your Mac…” with a time-remaining countdown. When this finishes, the system will show you the results of the test.

Interpreting the Tests

Hopefully the tests determine your system is clean and functioning properly; however, if not, then they will display an error code such as the following, to indicate a problem:

4SNS/1/1/4000000 TL0P-130

While this code might seem unintelligible, parts of it can be broken down to determine where the problem lies. The following list of codes show the possible outputs that you might see from the hardware test:

Hardware test results

The results of the hardware test, if any, will show here (click for larger view).

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

From this list, you can see the “4SNS” in the example above means there is a problem with a system sensor, which can be a temperature sensor, a current sensor, or a voltage sensor. Now you can determine which of these is the case by looking at the final detail of the reading (TL0P-130). In this case “T” stands for temperature, but it could also be “I” for current, or “V” for voltage.

The letter following the “T,” “I,” or “V,” lets you know what component of the system the sensor is in. The following list is a breakdown of what each of these components is:

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

From this detail, we can see the example problem is with a temperature sensor in the LCD display of a MacBook system.

Actions to take

Unfortunately with hardware errors, there is often not much you can do except take the system in to an Apple store to have parts replaced. While some components such as memory and hard drives are user-serviceable in some Macs and can be swapped out, for other components this is not easy to do.

You can try resetting the PRAM or reset the system management controller to see if that helps clear the problem. Otherwise, if the problem is with a Wi-Fi card or optical drive then you can try opening the system to see if relevant components have become dislodged, and then reseat them.

If you are unable to perform these tasks, or feel uncomfortable with opening your system, then take it in to an Apple Store or an authorized Apple Service Center for servicing.

13 thoughts on “How to run and interpret Apple’s Hardware Tests on your Mac

    1. Topher Kessler Post author

      They are different. These tests are for various sensors throughout your Mac’s hardware, whereas TechTool just focuses on memory and hard drive health. Apple’s hardware tests will not check your file system, but will see if voltage or current to a specific component is too high or low. As for the memory test, I am not sure which is more thorough, but sometimes one may find a problem where another one fails, so I often recommend to use multiple test suites for checking RAM.

  1. Charles Lewis

    My current Macbook Pro was originally purchased with Snow Leopard installed. It now has Mavericks. I still have the 10.6 disc. Would you suggest for or against using it to perform this diagnostic with 10.9.2 installed?

    1. Topher Kessler Post author

      The hardware diagnostics tests are separate from the operating system. The one on your 10.6 restore discs will be adequate for your Mac’s hardware.

  2. Javi González Roces

    What if you cant get any errors from AHT? I’m running complete test, and, when it’s almost done, it stoppes suddenly, before i can see any result :( And i dont find info about this.

  3. sjc

    Hey Topher, on my MacPro the power light flashes and it never gets to the chime. So no booting at all until I took out 6 of my 8 4GB sticks and it starts up fine. How do I determine which of the other 6 are bad? Do a pair at a time and when it won’t boot again, replace that pair? Thanks.

    1. Topher Kessler Post author

      This suggests a motor problem (MOT) with your hard drive. As with any sensor reading, it may be in error, but you might consider having your drive replaced. Regardless, be sure you make regular and ongoing backups of your data.

  4. diego temoche


    I have a macbook pro late 2011
    it keeps turning off after 20 minutes using it. it would go off even with the battery full.
    It would turn back on normally but it would always shut off after 15 to 20 m . i runned the hardware test and this code popped up.

    ASNS/1/C0000008: TPCD–124




    what is CP for?

    thank you so much for your info

    trying to fix it myself

  5. Susie

    Received an error message of: 4MEM/63/0000000:0x7f52c590
    Ok, I understand it is a memory error. I do not see the lower case ‘x’ , ‘c’, or ‘f’ in your list above. Is there anything else you can tell me about this error message?
    Any assistance would be greatly appreciated.

    1. Topher Kessler Post author

      While the nuances of the problem might give some insight, overall this means your system’s RAM is not functioning correctly, and will need to be replaced. Regardless of the specific symbols for diagnosing this, at its current state your system will likely be unstable and hang and crash until you replace your RAM.

  6. kerry

    hi. I’ve had a 2007 iMac (Apple refurb) since 2009, run brilliantly until 2 months ago. Started freezing up with no apparent cause. No issues of slow running and runs and performs fine until it freezes. Sometimes runs for 6 or more hours other times only 1 or less. No obvious pattern. Had a Genius Bar appointment and they ran the tests and found no faults so said it was a programming issue not hardware. Tried a fix but it didn’t work so did a complete wipe and reinstall. Said it would be ‘as new’. It froze first time I used it, issue not resolved. Not been able to take it back yet, was getting the spinning beach ball of death with whole lot freezing sometimes, other times the curser/ window could be moved around the screen but no actions working. Now started getting the white screen and flashing folder with ?. Used the DVD boot option to get it going. Ran Apple Hardware Test from disc and it froze while doing the long test, it was very hot by now. Restarted it later and ran both short and long test, no issues found. I heard the fans start up and run normally during test but they haven’t run like that for a while during normal use. Is this an incompatibility in programming as it is 8 years old or is it a parts failure? Is it worth doing another reinstall. Any help appreciated thanks.


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