Hampton Bay Dehumidifier Model# HB50

I'm having a problem with my Hampton Bay Dehumidifier. It's about 3-4 years old, so it may just be showing its age. But suddenly in the last week, it won't shut off. It starts up as soon as I plug it in, even at the lowest setting. I have same unit and exact same problem, as soon as unit is plugged into the wall outlet, it kicks on and no matter what I set the humdistat to even off, it continues to run. the other switches all work, high/low fan speed, the float switch, just the humidstat dial has no effect.