Raspberry Pi Act LED Error Patterns

If a Raspberry Pi fails to boot for some reason, or has to shut down, in many cases an LED will flash a specific number of times to indicate what happened. The LED will blink for a number of long flashes (0 or more), then produce short flashes, to indicate the exact status. In most cases, the pattern will repeat after a two-second gap.

Look up the code on the official documentation page: LED warning flash codes.