Performed a heating pm on (3) RTU’s. Replaced the air filters, checked the belts. Inspected the heat exchangers. Cleaned the flame sensors. Checked the electrical connections and safety’s. Checked the gas pressures. The middle unit(office), the blower was cycling on and off several times after turning the power back on. Checked the wires and connections. Reconnected the plug going into the gas relay board. After doing that the system started to work properly. There is a brown spot on the board next to a couple of resistors, if the problem occurs again, recommend replacing the board.