Once the CPU has been given the power good signal, the system firmware performs a power-on self-test (POST). The POST is a diagnostic program implemented in the system firmware that checks the hardware to ensure the components required to boot the PC are present and functioning correctly. On modern computers the POST happens very quickly to improve boot times, so you are unlikely to see any POST messages.

If power is present (you can hear the fans spinning, for instance) but the computer does not start, there is a black screen, and there are no beeps from the internal speaker, it is likely either that the display is faulty or that the POST procedure is not executing. Assuming you can rule out an issue with the display, to troubleshoot POST, try the following tests and solutions:

1. Ask what has changed—If the system firmware has been updated and the PC has not booted since, the system firmware update may have failed. Use the reset procedure.
2. Check cabling and connections, especially if maintenance work has just been performed on the PC—An incorrectly oriented storage adapter cable or a badly seated adapter card can stop the POST from running. Correct any errors, reset adapter cards, and then reboot the PC.
3. Check for faulty interfaces and devices—It is possible that a faulty adapter card or device is halting the POST. Try removing one device at a time to see if this solves the problem (or remove all non-essential devices, then add the back one by one).
4. Check the PSU—Even though the fans are receiving power, there may be a fault that is preventing the power good signal from being sent to the CPU, preventing POST.
5. Check for a faulty CPU or system firmware—If possible, replace the CPU chip with a known good one or update the system firmware.

If POST runs but detects a problem, it generates an error message. As the fault may prevent the computer from displaying anything on the screen, the error is often indicated by a beep code. Use resources such as the manufacturer’s website to determine the meaning of the beep code.

The codes for the original IBM PC are listed below:

1 short beep                         Normal POST—system is OK. Most modern PCs are configured to boot silently.

2 short beeps                       POST error—error code shown on screen.

No beep                                Power supply, motherboard problem, or faulty onboard speaker.

Continuous beep                  Problem with system memory modules or memory controller.

Repeating short beeps         Power supply fault or motherboard problem.

1 long, 1 short beep             Motherboard problem.

1 long, 2 or 3 short beeps    Video adapter error.

3 long beeps                        Keyboard issue (check that a key is not depressed).

Some PCs will not boot if a key is stuck. Check that nothing is resting on the keyboard. If the board is clogged with dust or sticky liquid, clean it using approved products, such as swabs and compressed air blowers.