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I will approach this situation from an applied physics point of view... my first assumption is that the printed circuit board has a microscopic crack in one of the thousands of circuits inside the printed circuit board and micro processors, these are almost impossible to trouble shoot and the strategy I employ is not found in any service manuals.
We can do a heat stress test on it.
Use a hair drier to slowly warm the printed circuit card up to120F or a bit warmer, but not over 140F... if there is any change in the error codes or other furnace behavior... the card has a cracked micro circuit that opens and closes as the card expands and contracts... that proves a bad printed circuit card.
No change however does not prove the card is good however... but achange proves the card is definitely bad.
We can go from there.
I see you have already replaced the board, however the flaw could be at one of the molex connectors, the heat stress test will show many (but not all) of those defects up.
The heat stress test is for the molex connectors at the board.
tell me please if that looks like the manual for your boiler or not. (all of us refer to manuals, auto repair shops have a wall filled with manuals, no one can remember every detail of all the equipment that has been made, it changes every year or two).
You currently have a retired mechanical engineer on the line, with 56 years experience and consulting nationally for fortune 500 companies nationally, I stand to make nothing by working with you, my statistics here for the last 6 years are 98.8 success rate... you are welcome to blow that off if you like.
Let me know however if you would like me to spend a little time with you on this, for close to 1/10 of what one of those service calls has cost you.
If you do the heat stress test I think you might find the condense sensor plug in to the printed circuit board does erratic when warmed up... if that turns out not to be the case, following that wire to the condense sensor itself, and cleaning or replacing it has a good chance of solving the problem.
We can go from there if you wish, or I can opt out.
I tried to send you a mark up of the wiring diagram on page 29, reference to the CS sensor but that screen shot service appears to be bogged down with holiday traffic, you can find it there however.
It is shown on the lower third, right hand side of the printed circuit card, the symbol shows it as a rectangle with a large curved hook shape on top.
After 4 years it is also likely that the condensate trap needs to be removed and cleaned instructions for that are on page. 22.
There is no problem with that, post when you can,
Hello again, if the heat stress test is what you are talking about, and you got those changes when you heated the printed circuit card up...you have proven the printed circuit board is bad, and it needs to be replaced.
If my test proved to be what led to that discovery, I will appreciate a positive rating, rating will not increase your charges but is how Just Answer keeps score here, it is important that I keep mine as high as possible. Thanks.