Intermittent shut down before ignition. Aire-Flo #AF80MPA075BSB

[Mac63]

Retired auto technician tackling my first furnace diag. if not allowed please remove, just trying to save my Girl some money here in these tough times, thanks.
Problem: Therm calls for heat, inducer motor starts, runs about a minute or two then shuts down with no sign of igniter lighting up. On the next cycle it may start normally, or it may not, it seems to repeat the no start sequence more often the colder it is outside. it has never exceeded 5 times with no start and will always eventually start.
My first look: I doped out a bad capacitor and a bad roll out switch with my Fluke. The roll out switch was in the mega ohm range. Replaced both and problem continued with same frequency.
My second look: I was able to get a code off the circuit board, code 3. Pressure switch stuck open or closed.
Took voltage reading off pressure switch. Get 28v to the switch on call for heat, inducer runs and switch closes i get 28v the other side of switch and all the way to circuit board. Bypassed switch in proper sequence, waiting for inducer to run then jumping, problem continues. No difference.
Put a Manometer on the inducer cover nipple. pressure switch for closing pressure is .35, manometer reads .7 when inducer comes on.
Took a step back: shut off main power and turned back on after a few minutes, circuit board holds and continues to read code 3. it doesn't clear and it doesn't require the system to self check before code sets. its a hard code.
Began process of checking inputs and output voltage of circuit board. While moving wires around with thermostat off, system went into start up. It also began flashing code 6 on its own. Code 6, roll out switch open. I cycled main power. Now when i pull off the wires on the roll out switch the furnace will start by running inducer motor and then shutting down before ignition begins. Power off i checked both Limit, aka, roll out switches and the high limit switch. All tested good, no resistance and no bad wires to circuit board.
Power back on, code 6 is gone and code 3 remains. BTW, code 3 did not occur while code 6 was flashing.
System is now running normally for the past 36 hours. Afraid to touch anything because she needs heat and it would take 3 to 6 days to get a board. My question, sound like a bad board to you or am i missing something?
Thanks so much for your help.

[Admin]

You did a good job!

My first thought was a pressure switch problem, but you bypassed it and still had the same problem.

Is the roll-out switch a manual reset?  Normally there is a button in the middle to press to reset it.

It's possible the pressure switch is sticking closed, which will prevent another heat cycle from starting.  It may be a faulty control board as well.  Worse case is there is a hole in the heat exchanger that is causing the pressure switch and roll out switch problems.  In colder weather the furnace will run longer and any cracks can open up.  Do the flames seem to distort when the blower kicks on?

I would make sure the furnace drain line is clean and clear.

[Mac63]

Hey thanks, I know it takes a long time to learn this stuff so I appreciate the compliment.
Yes the roll out switches have resets, I'm confident I've ruled those out. Since I posted I ran and got a pressure switch, cheap enough, and it makes no difference in the way it acts.
So, I haven't paid much attention to the heat exchanger and I am brushing up on what to look for on the next visit. From what I recall in my memory, we have a problem there. Seems as though I recall seeing the flame roll all the way across the end of the flame box instead of being confined to the tubes. This unit is about 15 years old and confined in a closet for a slab home. My thinking is that without being able to breath well in the confined space everything is failing on being overheated including the circuit board. The act of checking voltages on the board caused different behaviors in the system. Either I have a bad connection, bad solder joint or a failing chip or relay. Sound Plausible?
Thanks for your help and I'll send pics of the exchanger next time I post. She does have a CO2 detector btw

[Mark3577]

I'm not a skilled furnace worker, but just had an intermittent furnace problem. It turned out to apparently be the igniter (I just replaced it, the furnace is working, and I'm still keeping an eye on it). It's a Trane XV90 and about 16 years old with the igniter never replaced until now.