Depending upon the level of leakage, (and NOT a full blown short), if it were to occur in one of those diodes, (which I have experienced in logic board and other electronic repair), that could definitely pull the 24v supply to the relay, down below the 18 volts required to fully close the relay contacts; since the diodes are wired directly across the relay coil terminals. Without necessarily destroying the driver transistor, although it may become overheated; and eventually fail. I am just trying to cover all of the bases here, as it appears he HAS found, so far, at least one of the relay coil windings to be open. I am just giving him further things to check while he has the relays removed, and awaiting replacement parts, as there may have been a surge to this unit, that opened the relay coil windings. Without an explaination. Also considering that the control PCB has a SMPS. Otherwise, he may replace the relays, and then still find that the unit does not work properly.