Before I condemn a circuit board, lets verify your ground connections at the board to furnace and furnace to ground incoming...
I would like you to take line voltage and low voltage readings to neutral and then to ground an compare. Take low voltage to C or common and 115v to neutral and then to ground.
Thank you very much for the excellent support. It appears it will be a new circuit board is in your future.
Also the inducer mounts will come out of tapped holes so you dont need to worry about losing any nuts.
This may help also - http://www.cozyparts.com/Lennox-Parts/forum/Lennox_service_manual/Furnaces/G40UH_G40UHX_09-01-2006.pdf
See link - http://www.climatedoctors.com/items/item.aspx?itemid=1785842
As far as the inducer motor sir, I try to refrain from hypothesizing. I have oiled them and had them work just fine....and totally wasted my time on some.
Motor - http://www.cozyparts.com/Lennox-Parts/ProductInfo.aspx?productid=24W95
It is ongoing for me for the amazement......
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.
And now ......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 doesnt "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.
Did you have this to install?
What do the diagnostic lights say?
As a test...remove the 2 wires from the pressure switch. Start furnace...after the exhaust fan starts touch the 2 wires together and hold together...they must maintain metal to metal contact. Tell me if operation still faults.
That makes feel bad as well. That is why I hem and haw and try to check everything before buying parts.
The good news.....you found it. I highly doubt a tech would have caught such a intermittent fault.