I have an American Furnace which back's up a heat pump. Model UD0C936A Originally it would not light up at all. The draft fan would run continuously. I checked all of the safety's except the flame sensor which I have no idea how to check. Since I could not see how the draft fan would run continuously unless the relay on the control board was stuck closed I assumed the board was bad. I obtained a new one and now it lights up. But after running a few minutes it will go out and restart, you can hear the gas valve click when it goes out. Sometimes the gas valve will click several times in a row very quickly and the flame will sometimes stay lit, sometimes go out. Sometimes it clicks only once and the flame goes out. Then it restarts fine, the ignitor glows, it restarts and goes through the startup cycle. It may run long enough to satisfy the tstat sometimes it quits and the board gives two flash's which means "System Lockout (Retries or Recycles exceeded)". It will ususally not go out if nights are in the upper 30's but now that the low is under 20 it goes out every night. That is probably just the amount of time it runs which means it has to restart more often. I am thinking either the new board is not giving the gas valve enough current, or the gas valve is weak. How do I check this?
I tried to run it with the high limit disconnected, the flame sensor disconnected, and the pressure switch disconnected. It did not react the same and gave codes which indicated that the appropriate sensor did not work. They all have good continuity.
Update, I put a voltmeter across the gas valve terminals, and also to the red wire and ground. Across the gas valve I get .17 volts, and to ground just under 24V AC. The voltage stays steady across the valve and too ground when the gas valve clicks off. So does this mean the gas valve is bad?
Last edited by H. Phillips; 01-05-2010 at 01:36 PM.