I was intrigued by how a simple thermocouple generated magnetism.
Then the flame sensor with ac to dc rectification
Ultraviolet cadmium cells that "see" flame
I have units that when they fail, they call or email me. Not the company, not the maintenance man. The unit itself will fault and call and tell me what it faulted on.Then I call its owner and tell them.
As far as if the circuit board is not the fix. We have a board we have inputs. When an input doesn’t "fit" it creates the lockout function. Now, we have 2 ways to cause this lockout negative input, or internal failure. By checking all the limits, sensor, ground, pressure switch etc we eliminated inputs.
Now we move on to internal failure. If the new board also failed it just means we did not identify the negative input yet. And given the process so far, I am confident the inputs are reporting positively.