POST (power-on self-test)

In short: The POST is the self-test the firmware performs right after power-on. It checks whether CPU, memory, graphics and basic devices work before the operating system starts.

In more detail: If the test passes, startup continues, usually with a short beep. If it fails, the computer reports the error via beeps, diagnostic LEDs, a small display (debug LED) or a text message.

In Depth

Typical checks: processor, chipset, memory, graphics, keyboard, drives, temperature sensors.

Common beeps (differ by manufacturer, always check the motherboard manual):

Pattern	Meaning (often)
1 short	POST passed
Continuous / endless	Power supply or memory problem
1 long, 2 short	Graphics card
3 short or repeated	Memory (RAM error)
No beeps, no picture	Power supply, CPU, motherboard, no speaker

• Debug LEDs: Four LEDs “CPU – DRAM – VGA – BOOT” show where the test stopped. If “DRAM” lights, reseat or test the RAM individually.
• POST codes: Two-digit hexadecimal numbers on a display on the motherboard (for example A2 = detecting drives, AA = startup complete). The table is in the manual.
• Typical causes and remedies: RAM not seated properly (reseat), empty BIOS battery (settings lost), graphics card without power cable, defective power supply, shorted front-panel cables.
• Faster startup: Fast boot shortens the POST.
• Afterwards: After a passed POST the firmware looks for the boot device.

See also: Boot process of a Windows 11 computer, CMOS and BIOS battery, Power-good signal