Performed diagnostic on York package unit. System is not cooling. Cycled thermostat for cooling. Unit kicked on for a minute and then shut off, tripped on low pressure switch. Gauged up to unit and found it low on refrigerant. With approval charged system back up to proper charge of refrigerant and added leak sealant and dye. Also found that thermostat was cutting command to unit too soon before reaching temperature. Replaced thermostat. System is now producing proper temperature split and cooling correctly. Cycled system, system is operational upon departure. Office to bill.