Some manufacturers like EPoX and ABIT are marketing motherboards with a diagnostic LED display soldered directly to the motherboard. This display operates in the same way as diagnostic cards sold on the market. If the computer does not turn on or freezes during start up, we just have to get the number that is displayed and look it up in the handbook to find the probable cause of the trouble.
Some other manufacturers, like DFI and MSI, use a more simple diagnostics device made of four LEDs. The principle is the same, if you computer doesn’t turn on or freezes during start up you just have to take a look at the diagnostics LEDs and see on the motherboard manual what is going wrong based on which LEDs are turned on and which LEDs are turned off.