The possible reasons：

(1) The fuse on interface board burns out.
(2) The data line connecting interface board with main board is damaged.
(3) Mashgas in transformer accumulates to push forward mashgas relay to motion.

Suggested solutions:
(1) Inspect and exchange the fuse.
(2) Inspect and exchange the data line.
(3) Open the vent of transformer and exhaust gas; if this fault occurs again, then the  reason may be that there is localized breakdown inside the rectifier transformer, oil withstand voltage declining breakdown or other abnormities. Contact the manufacturer and then settle the problems. （lq）