York YAEP chiller problem- power failure alarm

Hi,
I have the problem still not identified. Could someone please tell me how to solve it.
Chiller- ambient air cooled 2 circuit York YAEP, China

Chiller is stopping immediately after starting the circuit. either 1 or 2. Alarm is also not the same every time. Changed the power board, processor board, relay board. All the fuses are checked ok.

alarm - power failure manual reset
or unit 1/0 switch in off condition.
and sometimes display goes out along with the above fault.

Suspecting there is some field side common circuit is dropping the voltage but where to check? What are the check points on the processor board
Anyone please guide

Thanks