What Is POST?

Definition of POST and an Explanation of Different Types of POST Errors

An example screenshot of a POST screen that says Operating System not found
POST (Power On Self Test) Example.

POST, short for Power On Self Test, is the initial set of diagnostic tests performed by the computer right after it's powered on.

Computers aren't the only devices that run a POST. Some appliances, medical equipment, and other devices also run very similar self-tests after being powered on.

You might also see POST abbreviated as P.O.S.T. but I don't see that too often anymore. The word "post" in the technology world also refers to an article or message that's been posted online.

POST, as is explained in this article, has nothing at all to do with the Internet-related term.

The Role of POST in the Startup Process

A Power On Self Test is the first step of the boot sequence. It doesn't matter if you've just restarted your computer, or if you've just powered it on for the first time in days. The POST is going to run, regardless.

POST doesn't rely on any specific operating system. In fact, there doesn't even need to be an OS installed on a hard drive for the POST to run. This is because the test is handled by the system's BIOS, not any installed software.

A Power On Self Test checks that basic system devices are present and working properly, such as peripheral devices and other hardware elements like the processor, storage devices, and memory.

The computer will continue to boot after the POST only if it was successful. Problems can certainly appear after the POST, like Windows hanging during startup, but most of the time those can be attributed to an operating system or software problem, not a hardware one.

If the POST finds something wrong during its test, you'll usually get an error some kind, and hopefully, one clear enough that you'll have a place to start troubleshooting from.

Problems During the POST

Remember that the Power On Self Test is just that - a self-test. Just about anything that might prevent the computer from continuing to start will prompt some kind of error.

Errors may come in the form of flashing LEDs, audible beeps, or error messages on the monitor, all of which are technically referred to as POST codes, beep codes, and on-screen POST error messages, respectively.

If some part of the POST fails, you'll know very soon after powering on your computer, but how you find out depends on the type, and severity, of the problem.

For example, if the problem lies with the video card, and therefore you can't see anything on the monitor, then looking for an error message wouldn't be as helpful as listening for a beep code or reading a POST code with a POST test card.

On Mac OS computers, POST errors often appear as an icon or some other graphic instead of an actual error message. For example, a broken folder icon after starting up your Mac may mean that the computer can't find a suitable hard drive to boot from.

Certain types of failures during the POST may not produce an error at all, or the error may hide behind a computer manufacturer's logo.

Since issues during the POST are so varied, I wrote a troubleshooting guide specific for them. See my How to Fix Stopping, Freezing, and Reboot Issues During the POST article for help on what to do if you run into any trouble during the POST.

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