I can offer some general information : most blower capered controlls use a resistor cluster in series to drop voltage for lower speeds , o if it only works on high , then thi is likely to be the issue. Again generally speaking theses resistor clusters are typically mounted in he plenum so that the. Blower air runs across them and cools th e resistors. If theses r the case I would also inspect the fan for leaves And or rodent nests. Typically what leads to the failure of h resistors