What is the function of the LED on the sensors?

The LED on the sensors is switched off during operation to save power. If it is flashing, it means that the sensor is too far away from the base station or not connected to base (either because it is offline or if the sensor is not connected to the base station).

The sensor LED also indicates the progress of the login process with base: if you hold down the pairing button on base as well as the one on the sensor simultaneously, both base and the sensor start to flash. When the flashing stops, you know that the sensor is logged in to base.