Hi,
I would start measuring voltages on both marked connectors on the attached image.
You should see the reference voltages written on the board. I marked those as well. Keep in mind that both connectors have two rows of connections so don’t just measure the ones on the top. You might need to disconnect the cables from the LED and the main board side to probe them with your multimeter.
[image|2322383]
You should have stable voltages and they should not be changing.