FAN REVERSE POLARITY
If the fan is plugged in backwards, this is the LED code that will apear. The code sequence loops LED6, LED5, LED4, LED3 constantly until the fan is unplugged. This will keep the ESC from arming and going to neutral and could appear if the fan is shorted or jammed.
The same code or a reverse version may appear when the ESC is switched on, but the receiver is not powered up or bound to the radio. Make sure the receiver power light is on!