Measure the input voltage of the main board with the input plug plugged in and removed.

• If the voltage always reads 24V, replace the main board with a new, identical one.
• If the voltage reads 24V with the plug removed and reads a different value with the plug inserted, remove all plugs other than the power input plug on the board and measure the input plug again. 

1. If the voltage now reads normally, one or more of the components on the mainboard is faulty. Plug the components back one by one to sort out the faulty ones. 
2. If the voltage still reads weirdly, the mainboard is faulty. Replace it with a new, identical one.