I've recently begun having issues with my Trane XR90 furnace and have discovered several similar issues spoken about across the web on various forums. The fix seems to be varied as well. Here is my situation:
I first noticed the furnace would come on, heat up to almost the thermostat setting (within 1-2 degrees), then cycle off, but never cycle back on. What DID keep running is the inducer motor. Got the 3 blinking light error - pressure switch error.
Additional issue that puzzles me: The inducer motor runs all the time. Even with the thermostat set in the "OFF" position, the inducer motor still runs. The only way to get the inducer motor to shut off is to either take the door off the furnace, tripping the safety switch OR simply shut power off to the whole unit.
Now, starting with master power off to the unit, if I turn power on thus starting the inducer, but have the thermostat in the "OFF" position then switch the thermostat to "HEAT" it will not fire.
If I leave the thermostat set to "HEAT" (or set it there first) THEN turn power on to the system, it will fire. If the furnace has been running and I turn the thermostat to "OFF" the system will cut the flame and blow the heat out as normal, but continue to run the inducer. In fact, this is how I first discovered the issue.
Based on things I've seen on various forums to try I've done the following:
Checked input and exhaust pipes for obstruction - clear
Checked the various condensate drain tubes in the system - clear
Pulled both the input and exhaust pipes from the system and let it run - burner still kicked off upon reaching therm setting and didn't come back on.
Checked the small tubes running from both sides of the pressure switch, one clear tube goes to what I think may be the bottom of the heat exchanger? The other black tube goes to both the gas valve and the top of the heat exchanger?
I didn't actually pull the clear tube 'cause it's hard to do with the pressure switch still mounted, but it doesn't look obstructed.
I did, however, separately pinch both tubes leading from the pressure switch. The black tube coming off the front did nothing. The clear tube off the back of the switch, when pinched and held for a few seconds causes the system to cycle back on - ignitor glows, then burner lights as normal.
I also tried removing the two leads to the pressure switch, then jumping those leads with a piece of copper wire. Nothing.
One thing that seems to have fixed one person's pressure switch error was a tech dialing back the pressure on their gas valve. This I didn't do, because I don't know how. There is what looks like a brass slot for a screwdriver on top of the gas valve, but I don't know which way to turn it and didn't want to start messing with that until I gain an education on what it might do. Is it unsafe to adjust that a small amount?
There IS good exhaust outside the house, so I know the inducer/exhaust motor is blowing well.
I thought I might just replace the pressure switch if it was inexpensive, just to eliminate a variable. But it looks to be somewhere between $65-85. So, I don't want to do that until I know. Too much money to guess on.
I sure appreciate any insight.