Duo Therm furnaces are used in recreational vehicles or trailers to supply heat through a combination of gas and electricity. If either one of those elements is compromised or your furnace's settings are incorrect, there is a chance your Duo Therm furnace won't light, even if a flame is present in the pilot burner.
Trailer Power Supply
Your trailer's power supply can affect the ignition in your Duo Therm Furnace. Electrical power to the Duo Therm is supplied by the trailer's battery. If the trailer's battery has a low voltage supply, the electrical spark needed for combustion is limited. If the connections from the battery to your trailer's motor are loose or damaged, the battery may not be receiving a proper charge.
The gas supply to the Duo Therm is electronically controlled during a start. The solenoid valve opens the gas valve when power is applied. Use a multimeter and follow your wiring diagram to see if it's still receiving the right amperage or has resistance. However, it's possible that the gas supply from the trailer house to the Duo Therm is compromised. Inspect the gas tank's shutoff valve to see if it's open or if there is damage. Also inspect the supply line to the Duo Therm for leaks, especially at the connecting point.
The Duo Therm furnace uses a spark ignition system in which electricity ignites a fuel vapor from the pilot burner within 30 seconds of operation. Any damage to the lead wires from the sparker could provide an intermittent spark, thus inhibiting combustion. Cracks in the ceramic housing can let the vapor escape before ignition as well. Inspect the pilot burner for excessive soot or damage, when can come from consistent operation at low temperatures. The carbon deposits can clog the burner's output point and interrupt combustion.
Your thermostat must be turned on for your Duo Therm furnace to switch back on after its initial start. If the thermostat is turned off during operation, it won't know to come back on once the original temperature setting is reached. Some models have a safety setting that locks out the gas valve once two failed starts occur. You'll have to reset your ignition settings according to your model's operating instructions.