A few things may cause this. 

One, it may mean your battery voltage is too low. Follow the instructions to check the battery in The station screen is showing an “L” or “X” where the battery indicator is. What’s wrong?

If you are seeing the red light while using a weather or distance module with your station, make sure that the sensor board used within the instrumentation (this is the board that is inside the Stevenson screen for FieldKit Weather or connected to the ultrasonic rangefinder for FieldKit Distance) is plugged in and all cables are seated correctly. A faulty connection with the sensor board will cause a red light to appear on the module board.

Finally, if the battery voltage is high enough (above 3.6 V) and you don’t have a connection issue as described above, go to the Modules menu in your station screen, and page through using the up and down buttons to see if any of the modules shows “fatal”. If so, the module may need to be reprogrammed. If this is the case, press the center button, erase the module, then reprogram it with the type that it is (e.g. for EC pick “water.ec”, for T pick “water.temp”, etc.). NOTE: After doing this, you will need to recalibrate your sensors.