DIAGNOSIS The variable speed switch inside the motor housing has failed or is jammed.
FIX Unplug the unit. Try rotating the dial through its full range several times quickly. Sometimes this can clear minor carbon buildup on the switch contacts. Plug back in and test.
IF THAT DIDN'T WORK The electronic speed control board or the potentiometer (the dial's internal component) is defective. This is inside the sealed motor housing. The motor unit must be replaced.
HEADS UP Running constantly on high speed generates more heat. If the thermal fuse hasn't blown yet, limit use to very short bursts until you can replace the unit to avoid motor damage.