What tool can discover and report computer errors and conflicts that occur when you first turn on a computer and before the operating system?

The answer is a POST Diagnostic Card.

Although not a necessity, a POST card can help discover and report computer errors and conflicts that occur when you first turn on a computer and before the operating system. To be able to understand how a POST card works, one needs to be very familiar with the BIOS. The power-on self test (POST) is a series of various tests performed by a computer when you turn it on. If you have any issues that conflicts with the operating system and prevents the computer from booting, you can install a POST card in the available expansion slots. This card will monitor the entire boot process and report errors in coded numbers on a small LED panel on the card.  

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