When a sensor is displaying FTST and no readings are shown, this indicates the sensor is in Factory Test Mode. A sensor will enter this mode when its silence button 
(bell symbol) is pressed and held for more than 7 seconds.
What happens during Factory Test Mode?
When a sensor has entered this mode, it performs a factory test and power-cycles (turns off and on), which can take up to approximately 1 minute. Once the factory test is complete, the sensor returns to its normal operating mode.
This mode does not impact the use of the sensor or the logged readings stored in its local memory. The purpose of this mode is for use by Primex during factory testing.