Upon arrival I was told Lobby unit was at 77 degrees yesterday, when I arrived thermostat was satisfied at 72. Went to roof and watched system operate. Ac cycled on so I checked operation, found no problems. System cycled on again and then satisfied. The 3rd time unit came on was in ventilation mode. I found supply air sensor temperature was fluctuating periodically from the correct temp to 25 degrees. Because supply air sensor was currently reading 25 degrees, the system shut off compressor and opened economizer 100%, letting all outdoor air into space. Shut off power and ohmed out temperature sensor, it started at 234ohms then went back down to 11.4ohms. Supply air temp sensor needs to be replaced. Removed economizer board plug and attached original plug to economizer, then went to damper actuator and removed plug and attached original jumper plug. System will not economize and bring in outdoor air until Sensor is replaced and we test to make sure program is working properly. Current