You may want to see if you have any codes in your climate control. That is B1. The fan speed is controlled by the power stage in response to signals from the actual climate controller, which in turn gets its signals from a myriad of sensors. So the codes are where to start. The default setting in certain conditions is max fan; in other conditions, no fan. Truly, the most likely suspect is the power stage a/k/a resistor. It is behind the glove box to the left of the blower motor...but first you need to rule out reasons for it to default.