Are you in an area with extremely low humidity? Just curious, as my outdoor sensor seemed to drop from 15% to nothing (don't remember if it was 0% or 1%) last month when we were having an unusual dry spell here in the northeast. It was sort of like the sensor was going out of range on the low side, and has been reading normally again since the humidity returned to normal for this area.
I suspect that you may be making a warranty claim, but you may want to peruse AcuRite's support forums first to see if anyone else reported the problem you're experiencing.....