Unit not cooling. Checked operation and found everything running as it should be. Connected gauges to check refrigerant pressures and found system low on refrigerant. Found nut on evaporator coil a loose. Tightened up nut and added 5lbs of 410a to system. System is cooling properly now. Let customer know when we come out for ac maintenance we will check pressures and if we loose charge we will add dye then when the weather is warmer out.