Jerrod, on our LM, holding the "down" arrow for three seconds will turn the rain sensor off and on. When the rain sensor is off, the small light will be on.
We discovered this process this weekend when, after three days of rain, the fan wouldn't open because the sensor was wet. Disabled the rain sensor, opened and closed the fan vent a couple of times, then turned the sensor back on. Works like a champ.
HTH.