model no: REYQ20 (combination of 2 REYQ10's)

Fired the VRV up last Friday and left operational. Back on site other day and found unit off on a U2 fault. Reset unit and operated. The system is only operating on the master VRV and the slave has not ran (may not being called for). As I understand, the U2 fault is power related, but all appears normal. Anyone shed any light on this matter?

Jase