BIOS Power-On Self Test (POST)

The first thing that the BIOS does when it boots the PC is to perform what is called the Power-On Self-Test, or POST for short. The POST is a built-in diagnostic program that checks your hardware to ensure that everything is present and functioning properly, before the BIOS begins the actual boot. It later continues with additional tests (such as the memory test that you see printed on the screen) as the boot process is proceeding.

The POST runs very quickly, and you will normally not even noticed that it is happening--unless it finds a problem (amazing how many things are like that, isn't it?) You may have encountered a PC that, when turned on, made beeping sounds and then stopped without booting up. That is the POST telling you something is wrong with the machine. The speaker is used because this test happens so early on, that the video isn't even activated yet! These beep patterns can be used to diagnose many hardware problems with your PC. The exact patterns depend on the maker of the BIOS.


Note: Some POST errors are considered "fatal" while others are not. A fatal error means that it will halt the boot process immediately (an example would be if no system memory at all is found). In fact, most POST boot errors are fatal, since the POST is testing vital system components.



Beep Code description of Problem:

No Beeps:                   Short, No power, Bad CPU/MB, Loose Peripherals
One Beep :                 Everything is normal and Computer POSTed fine
Two Beeps:               POST/CMOS Error
One Long Beep:      One Short Beep Motherboard Problem
One Long Beep:      Two Short Beeps Video Problem
One Long Beep:      Three Short Beeps Video Problem
Three Long Beeps:                Keyboard Error
Repeated Long Beeps:         Memory Error
Continuous Hi-Lo Beeps:   CPU Overheating