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The 3B2 computers have a pair of LEDs on their front panels, one green and one amber/red. When system problems crop up, the 3B2 may give a hint by flashing these LEDs, and this table summarizes what those LEDs mean. This is taken from the 3B2 Maintenance Reference Manual.

LED Message Meaning/action
1 amber/red pulse Unknown Problem: the console may be wired incorrectly or there may be other trouble not revealed by LED pulses. Investigate further using diagnostics.
2 amber/red pulses Problem with CPU
3 amber/red pulses Problem with firmware (ROMs)
4 amber/red pulses Problem with RAM. Possible bent pins in backplane (usually where system baords plug into backplane), defective I/O boards, or any hardware failure that may cause a problem with memory access.
5 amber/red pulses Problem with the DUART (console serial port)
Both green and amber/red LEDs on Reseat memory boards, I/O backplane. Remove feature cards. Try to reboot. Disconnect battery and reconnect (to clear CMOS), then reboot. Swap first memory card.