There are 2 LED light on the board to check the status.
D1 not work, D2 not work.
Test the “IC” , if the board work well, it should be around 3.3v.
D1 light on, D2 not work.
If your D2 LED not work when power on, your LED may dead (not effect your printing) or your “U3” chip is dead. （control LCD printing panel）
To check if the “U3″ chip is good. Test the C27 capacitance， connect one” – ” with C27 capacitance’s “+”, it should be around 5.96v.
Test the C61 capacitance， connect one” – ” with C61 capacitance , it should be around – 5.96v.
D1 not work, D2 light on.
If your D1 LED not work when power on, the D1 LED may damaged, or the board is dead(core ARM chip damaged)
Test resistance on D1 will know, usually, you can test IC (same as D1 and D2 not work) directly.