Mitsubishi city multi frequent control card issues

Hi all. I have about 30 condenser units of Mitsubishi city multi. Model is PUHY. Since past 6 months we have replaced the controller pcb for about 10 units. The symptoms are, the fcu will lost communication with the cu. Upon investigation, it's found the control pcb 7 segment display has no light but there is incoming power to the board at the fuse. Once board is replaced, the cu works fine.

Our system is 5 years old. So what is the main culprit causing the pcb to malfunction. Each board costs a whopping sgd633 o