HEY< that could be the trouble, before you buy a circuit board cause it sounds like its doing its job. Take the rubber hose aloose from the fan switch and and while its starting, put the hose in your mouth to feel a suctions if not or a soft suction , then remove the hose off the motor while off and clear the end out with something that ,ll clean the opening out and connect the hose back and start the system again. T Y V M!