How to Run Apple Diagnostics on a Mac and What the Results Actually Mean

Apple Diagnostics is the built-in hardware test on modern Macs. It takes a couple of minutes, it costs nothing, and it can surface problems with certain hardware components. I run it constantly. I also do not trust it on its own, and I will explain why.

• How to Run Apple Diagnostics

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Before you run it

Apple recommends updating macOS first if you can. Then shut the Mac down and disconnect everything except the power adapter, keyboard, mouse, display and Ethernet if you use it. Put the machine on a hard, flat, stable, well-ventilated surface. Leftover peripherals are the most common reason people get a confusing result.

Apple's own instructions for Apple Diagnostics worth keeping open while you do this.

Apple silicon Macs

Press and hold the power button, or Touch ID if that is your power button, until the startup options screen appears. Let go, then press Command-D. The test runs on its own and gives you a result when it finishes.

Intel Macs

Turn the Mac on and hold the D key right away. If that does nothing, restart and hold Option-D instead, which runs the test over the internet. Older machines sometimes only cooperate with the second method.

On macOS Tahoe 26 and later you may get asked which diagnostic you want to run instead of it just starting. Pick the standard hardware test unless you are chasing something specific.

What it actually tests

Apple says Diagnostics can help identify which hardware component may be at fault. It does not test external hardware, and it does not sort out software conflicts. So a Mac that behaves badly because of a bad app, a failing external drive or a messy install can still pass.

Why diagnostics matter more when buying a used Mac

When you are buying a Mac you did not own before, speed matters. You cannot verify the full history of the machine, and the seller may not remember every issue. Apple Diagnostics gives you a quick, consistent way to check whether the hardware is reporting a fault before you spend more time on it.

A clean result narrows the inspection, but it does not replace hands-on testing or checks on ownership and software state. In a resale or refurbishment environment, the goal is to reduce uncertainty before you price, repair or resell the device. The diagnostic is one input into that decision.

How I use Apple Diagnostics in a real inspection

1. Identify the exact Mac and configuration.
2. Confirm Find My / Activation Lock and management state are clear.
3. Run Apple Diagnostics when appropriate.
4. Hands-on test the display, keyboard, trackpad, ports, camera, speakers/microphone, Wi-Fi/Bluetooth, and charging.
5. Review battery condition where applicable.
6. Inspect physical condition and signs of liquid/impact damage.
7. Decide whether the device is ready for resale, needs repair, belongs in parts recovery, or should be passed on.

Activation Lock and management enrollment are not hardware faults, but they will stop a Mac from being usable by the next person. The diagnostic will not say a word about either one, so I check them before I rely on the result.

About those reference codes

If the test finds something, it gives you a reference code. Treat the code as a starting point for service, not a verdict and not a price. It narrows down where to look. It does not tell you what the repair costs or what the machine is worth, and I would not use it to value a device on its own.

If it passes and the Mac still acts wrong

Try to reproduce the symptom on purpose. Note what you were doing when it happens. Then separate software from hardware: restart, remove peripherals one at a time, and see if the problem follows a particular app or a particular port. If it survives all of that, take it in for service.

Run the diagnostic. Keep the result. Then keep looking.