When our Jabsco macerator stopped working, I bypassed the control box and put the 12v supply straight to the pump, and voila, it kicked in, so I put a switch in the supply and operated it manually, until the new control box came. Fitted that and everything back to normal. If the pump hadn't worked, I would have had a look at the impellor, a simple job. Usually though you can tell if it's the impellor by the pump making funny noises, or your lights dimming. By the way if you block the toilet it is quite easy to overheat the relay, causing it to stick and then the control box can't do it's stuff.