I have a Hydro Flame furnace, 8400 series, that was working fine then all of a sudden it shut down and continued blowing cold air. I've replaced the module board and the ignitor. The problem still persists. I can not get the unit to fire. I have researched this and have been told to check the sail switch. I have no idea where this sail switch is located. It has what the manual calls a "Johnson Gas valve". Is there a way of testing this valve? I removed the valve and cleaned the two coils, and blew out the spool of the valve. Is there a replacement? Someone said to check an orifice near the rear of the unit. I did not see any reference to this in the manual. Any Ideas??