Atwood 8531-IV DCLP Furnace Not Working

dougrainer wrote:
1. Jumping the 2 Blue input wires at a furnace does NOT BYPASS any Limit or safety control on a furnace
2. The 2 blue wires----ONE is output 12 volts to the tstat OR the AC control board. The OTHER BLUE wire is the down 12 volt supply from the Tstat OR the AC control board to tell the furnace to start a cycle
3. Colors other than BLUE are wiring the OEM runs from the furnace to the Roof AC control board or the Wall tstat.
4. You NEED to know what type AC brand and control you may have to troubleshoot.
5. Does the furnace 2 wires go directly to the Wall Tstat or to the Roof AC control board? THAT is needed to know to troubleshoot
6. IF you can get access to the 2 BLUE furnace wires at the furnace, you should have 12 volts on 1 Blue and nothing on the other. IF you have 12 volts at 1 BLUE then jump the blues, the furnace will come on and light and run as long as those 2 BLUE wires are jumped. IF the furnace runs and stays running longer than a few minutes, your problem is NOT the furnace but the Tstat control system.
7. IF the furnace fails to stay lit and run for a few minutes, your problem is in the furnace control and the furnace needs to be diagnois and fixed. Doug

Thanks for this explanation.

Furnace wires all go to the AC unit. I DO have 12v on one blue, but not the other. When I jump the blues, The fan comes on, ~15 seconds later the furnace sparks & ingnites and burns until I remove the jumper. When I remove the jumper, the flame goes out and the fan runs for a bit longer ~1min, then goes out. The furnace will continue to run as long as the jumper is in place.

When I have the jumper in place, The furnace runs regardless of the set point of the thermostat and whether or not the switch is set to "heat" or "off."