First of all, many thanks to Roughneck77 and PJMax for their wisdom. I learned a ton. I figured I'd post this so possibly it may help some others who have cranky old furnaces that behave randomly

My venerable American Standard (Trane) furnace decided that after 17 years of reliable service, to give me a hard time this fall. The AC side was spastic over the summer, and I suspected that old age was starting to set in, but I never anticipated this level of leprosy. Attic installed on its side, always maintained, but never really needed much.

When we started it for the first time, it exhibited random behavior. II went on, then off, It might blow cold air, then shut off, then went into protect mode. Sometimes, it would run but never shut off. Sometimes, it simply wouldn't start. It did this over and over and over again, in a variety of different patterns. It never behaved the same twice in a row, and diagnosing was/is challenging. It is really clean, orifices, filter, burners, etc.

One of the things that the AC side did this summer, was never shut off, even if I disconnected the thermostat. Same behavior with the heat when it would actually run. After a bunch of test measurements, I decided that for $150 clams, a new controller board would be prudent, as the original control board could be a culprit in the fan overrun problem, as well as random behavior. Simple enough, took about 10 minutes when I got the new White Rodgers replacement. I also checked everything from the rollout switches, flame sensor, pressure switch, etc. Checked voltage and impedance on all. Everything seemed on spec.

On first start after the new control board, it did its thing: I watched and timed each step based on the control boards documentation. Call for heat, Inducer fast, then slow, ignitor, gas valve, and bam, had flame, another minute or so, the fan came on, and we were in business! Until we weren't...

It ran well for the better part of the day, until it was cold... really cold. Back into the attic, did a reset with the master switch (AC) twice per the control board instructions, then checked codes, and all it said was "retry lockout". Oh joy. 19th century diagnostic codes in the age of digital controls....

So I power cycled it a bunch, used my phone to video record each try, and each time, it simply behaved differently. Some times, it would start, but not restart after the first time. A few times, it would start, then the flame would go out after a minute. Other times, the gas valve would click when it was its turn in the sequence, but no gas would flow. Other times, the inducer would go to low speed and just sit there until it timed out. Yet other times, it would start and run fine for a half day or so.

At one point, the ignitor didn't come on. Fortunately, I had a generic W-R that came with the control board, and I adapted it but ordered another silicon nitride wafer starter and replaced that. I also replaced the flame sensor, after a couple of times when the flame went out after a minute or so.

After a couple of weeks of continued oddball behavior and more video sessions, the gas valve appears to have given up the ghost after 17 years of yeoman service. So today, I changed that and it actually fired right up and ran fine.

I'm hoping that the series of cascading events that led to the random behavior has been rectified, but after a month of completely random behavior, I have a wait and see attitude.