POST, short for Power On Self Test, is the first set of diagnostic tests that the computer performs immediately after it is turned on. Its purpose is to check for hardware problems.

Computers aren’t the only devices that perform a POST. Some appliances, medical devices, and other devices also perform very similar self-tests after they’re turned on.

A Power On Self Test is the first step in the boot sequence. It doesn’t matter if you’ve just restarted your computer or if it’s the first time you’ve turned it on in days; the POST is going to run.

POST is not dependent on a specific operating system. In fact, an operating system does not need to be installed for it to run. This is because the test is handled by the system’s BIOS, not by installed software. If an operating system is installed, POST will run before it has a chance to boot.