I preformed a maintenance on 2 Ruud units. Indoor units: I checked amps on the blower motors and heat strips, I checked the electrical wires and connections, I checked the drain pans and drain lines, I checked the float switches, I checked behind the disconnects. Outdoor units: I checked amps on the compressor and fan motor, I checked the capacitors, I checked around the contactors and the boards, I looked into the units for rubbed wires and rust, I cleaned out the bottom of the units, I vacuumed the drain lines and washed the units down with water. When I started to check the outdoor units, I noticed the upstairs outdoor unit wasn’t on, I took the door off and noticed a flashing code telling me low pressure, I checked the two valves, I applied my gauges, I pushed in the contactors for a brief second, I then concluded that the unit had 0 refrigerant, Other than the upstairs outside unit everything is working properly, I spoke to both the homeowners and they would like it to be taken