I have a carrier 58RAV induced combustion furnace. It has been blowing cold air for a while now, and I am trying to fix it. The fan turns on but the flame never gets lit. The LED status light is blinking rapidly with no pause. According to the user manual this means... "Check for correct line voltage polarity. If units are twinned check for proper low-voltage (24V) transformer phasing" I do not know what that means. How is it possible for the polarity to change?