Trane XV90 second stage doesn't always work

[Mark3577]

Furnace was installed in about 2008. The circuit board was replaced about 2011. I had second stage problems again after that, a tech said it needed another circuit board. A second opinion tech got it running in 5 minutes and no circuit board needed. The second tech said there's something about these Trane furnaces and unfortunately I didn't hear or forgot, and I can't find that technician. The furnace has not been serviced in several years.

What mainly happens is the second stage doesn't kick in automatically. If I set the thermostat at 65 at night, then I'll wake up and the temp will be 62. A double red blinking light happens (lockout). I cleaned the filter even though it looked good. I've read about flame sensors, limit switches, etc. I've shut off and on the thermostat, power, and various combinations thereof.

But I can get the second stage to run sometimes. If the room temp is 62, then I set the thermostat to 62, turn off the power then back on, let it run at 62 for several minutes and watch the slow flashing green light, then change the thermostat to 70 and the second stage may kick in with fast green light (this has worked several times). So:

1. If anyone knows of a common quirk of this furnace (like that tech said there was something about what they tend to do), then any input would be great
2. I'm trying to understand, but mainly don't want a service tech to replace something, especially a circuit board, when it's not really needed.
3. If something is very likely the problem then I may DIY, but probably not. But it would be good to know for knowledge and discussion purposes with the tech. Thanks.

[Admin]

It could be a pressure switch issue.  A venting or drainage problem can cause this.  Check the venting slope and that the drain line is clean.  Also look for water leaking inside the furnace.  The secondary heat exchanger header can crack and leak and also cause pressure switch problems.

[Mark3577]

Thanks for the comments. A tech came out, opened the covers, taped the cover switch so the furnace would run, did his alligator wire connections, ran the furnace twice to check the 10 minute call to second stage, and said all was fine. It isn't fine.

The furnace still doesn't kick in automatically to second stage to maintain room temperature. It still gives two red flashes (evidently when it has called the second stage too many times). But second stage will work (one time) when I do a sequence as I originally stated.

I will try harder to get a more hands-on tech and give them your comments. Thanks again.