I am having the problem of my USSC 6220 pellet furnace shutting down with error code 2. This normally means that it's out of fuel. However it is not out of fuel, pellets are feeding fine and the fire is strong and hot now when I get this code. I have so far both jumped the vacuum switch or eliminated it using the C codes. Both to no avail. I have run the diagnostics and found that often when testing immediately right after it shutdowns that the thermostat in the plenum either reads zero or sometimes a negative degree or something out of the realm of possibilities. I replaced that thermostat with a new one, again to no avail. I have seen one zero reading since replacement. The furnace will run anywhere from a 1/2 hour to several hours after startup. This same scenario happened to me 2 years ago and lasted for a couple weeks and then as suddenly as it started, it stopped and has run fine for 2 years. Now the problem has returned. The only thing I can think of now is that the main circuit board may be bad. Ive done resets of the board and cleaned all the connections with contact cleaner. Any other suggestions?