The main board on the Original Prusa MK4 and MK3.9 is the xBuddy board. The xBuddy board has ten status LEDs.
Error lights
MMU overcurrent
The MMU overcurrent light indicates that the current through the MMU port is higher than 3.6 A.
Print USB overcurrent
The print USB overcurrent light indicates that the current through the print USB port (---) is higher than 0.5 A.
Heater fault
The heater fault light indicates an overcurrent through the hotend heater, higher than 2.8 A.
Current fault
The current fault light indicates an overcurrent through the whole printer, higher than 15.5 A.
Service USB overcurrent
The service USB overcurrent light indicates that the current through the service USB port (---) is higher than 0.5 A.
Status lights
Model Fan act
The print fan cools the printed model as it lies on the print surface. The associated LED indicates that this is active and spinning.
Heatsink fan act
The heatsink fan cools the hotend components, including the heatsink. The associated LED indicates that this is active and spinning.
Heater act
The hotend heater light indicates that current is flowing through the hotend heater. During a temperature rise, the light is fully on, while at a stable hot temperature, it is expected to blink.
Main power
The main power LED indicates the presence of the needed voltage for the xBuddy to operate, 3.3 V.
Heatbed act
The heatbed light indicates that current is flowing through the heatbed. During a temperature rise, the light is fully on, while at a stable hot temperature, it is expected to blink.