LB - Low Battery Condition
Check that the batteries are connected in the battery box, current units (M29T2106) are sold with the batteries disconnected.
Measure the batteries under load (should not drop below 22V DC), check the transformer (26-27VAC). *Should the mains fail, a double beep will be heard every 30 seconds to indicate mains failure.
IN SETUP:
If batteries are connected and the Error code still appear, check the DOSS connection on the PCB.