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Hi, unfortunately not all techs in any industry are created equal. In fairness, intermittent failures are the hardest troubles to find since when the device is working......there is nothing to fix.
So you have a new control board; everything worked OK for 24 hours; you have propane pressure to other appliances and they are all working OK and now it sounds like your furnace is getting all the way to the ignition cycle but not lighting and it is then going into lockout after three attempts to light. To restart you have to turn off the thermostat and wait 15 seconds then turn on again; OR cycle the on/off switch in the furnace. Did you smell any propane being released during the ignition attempt sequence?
So some possible failure issues that may be preventing ignition at this point in the furnace cycle (I wonder if any techs checked these items?):
1. While unlikely (infant mortality), the new Control Circuit Board “may” be a problem and either does not open the gas valve or does not trigger the igniter (but you said you heard the igniter sparking for the three attempts). Check for 12V at the gas valve (if you can) during the ignition cycle which confirms the board is sending the signal and the connection is good to the gas valve.
2. The Gas Valve fails to open, so no propane in the burner/oriface (no gas smell either after several start attempts). “Assuming” no issues with gas line feeder to the gas valve; replace the valve.
3. The Gas Valve is open OK, but no propane flows. You verified other gas appliances work OK so propane regulator is good, but is gas getting to the furnace gas valve? Unless the furnace feeder is restricted or kinked somehow the gas feed line to the gas valve should be OK (famous last words). Check possible dirt or bugs in the gas line or burner oriface after the gas valve. Clean out debris from the lines/oriface.
4. The Igniter fails to spark. Could be a defective igniter or no power is reaching it. It is also possible the gap at the ends of the Igniter electrodes is too large (or that they are shorted together). Inspect and adjust/proximity to gas oriface also.
5. Igniter does not confirm that ignition was successful (no flame is sensed by control board), so the Control/Ignition board has shut down (lockout stops restart attempts).
6. Propane and spark are present but the burner doesn’t light. This can occur if the Combustion Air Intake is clogged (mud duaber nest/spider webs etc.), and there is insufficient air flow through the combustion chamber.
7. Loose or corroded connections in any of the above components.
Not sure how comfortable you are with getting into this, but the tech(s) you hired should have gone through a process to repair your furnace. Intermittent troubles are the worst thing to work on though.
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