What Is POST?

Definition of POST and an explanation of different types of POST errors

POST, short for Power On Self Test, is the initial set of diagnostic tests performed by the computer right after it's powered on, with the intent to check for any hardware related issues.

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.

Number columns on a computer screen
The unrecognized ordinary / Getty Images

You might also see POST abbreviated as P.O.S.T., but probably not too often anymore. The word "post" in the technology world also refers to an article or message that's been posted online. POST, as it's 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 it to run. This is because the test is handled by the system's BIOS, not any installed software. If an OS is installed, POST runs before it has a chance to start up.

This test checks that basic system devices are present and working properly, like the keyboard and other peripheral devices, and other hardware elements like the processor, storage devices, and memory.

The computer will continue to boot after the POST, but only if it was successful. Problems can certainly appear afterward, 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 of some kind, and hopefully, one clear enough to help jump-start the troubleshooting process.

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 might 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. For example, one of the AMIBIOS beep codes is three short beeps, which means there's a memory read/write error.

If some part of the test 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 issue 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 computers, these errors often appear as an icon or another 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 might 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, you may need a troubleshooting guide specific to them—see How to Fix Stopping, Freezing, and Reboot Issues During the POST.

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