How To Fix Sega Dreamcast LED after LED Mod - LED won't turn on 

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Check that your ribbon connected to the CPU and controller port is in fully. Also check if the fan wire is connected securely. If either one of these are loose then the console won’t power on

@@Gamextc oh okey i Will check

The fan is spining but the led light is off

What was the rating of the LED that you used and has it been installed the correct way round? With a multimeter test the LED and see if you get a reading.