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Hi my name is Robert....
After the fan comes on does it try to spark at ALL???
IF it is sparking IT is NOT a problem with the sail switch, high limit or thermostat.
The fan will blow on that sail switch and when it closes it passes a signal to the circuit board, once the circuit board sees that signal then it will spark.
IF you had low voltage or a faulty sail switch it would never spark. IF you have power through the sail and high limit to the board and it is not sparking then you have a bad circuit board or electrode.
Make sure your battery voltage is good as well, I read the part about the new battery, if the voltage was dropping under 11.5 volts the fan may not spin fast enough to close the sail switch.
IF it is sparking please let me know, it matters on how to diagnose these if it is sparking or NOT.... Robert
What is the model of the Duo therm? I can send you a service manual, you need to see if power is getting to the circuit board via the sail switch, if you have no power then the furnance is not going to spark, if you have power then this is going to be a bad electrode or circuit board. Most RV shops have a special board tester and can tell if the board is bad or not. The tech should have tested the board out, but if he told you there was not a replacement for the sail switch then I would not count on anything he did.... Robert