As the wet side of Vortechs soak and get a bit of coraline or calcium, any momentary binding of the shaft can cause the controller to initiate a shut down and the blinking red light. Usually, it can be cleaned and reassembled and run for a long while longer. If it's really jammed up, the best thing to do is purchase a Wet Side Maintanance kit, comprised of a new prop, plastic bushing, nut and bolt. It's the bushing that wears out and will cause your Vortechs to start knocking or rattling. If they are not almost silent, it's time to tear them down.