If you are recieving that error you have a mini max NT or a mastertemp. Your in for a horrible tie trying to mess with that yourself, you should probobly call a service company. Repairing those heaters is a very inexact science. If your fans vaccum sensor is actually the problem, you will have to find which one you need, they are nnot interchangable. take off the two sides of your heater and ensure that nothing has been sucked into the draw line for the fan. Ensure you have your gas valve open. If you have had any new gas appliances intalled, that could be the problem also. Heater repair is very complicated, I would not reccomend getting involved with it. (if you do have a service company come out and repair it post whatwas wrong, about 50 percent of minimax NT repairs end in a new "fenwall")