Thanks for taking the time to read the post.
This is a trane xr90 gas furnace. The inducer fan runs constantly. The unit will run and meet the set point on the thermostat and then shut down except for the inducer fan running. The unit will not start again unless I manually reset the pressure switch - gently move air in the tube to the pressure switch.
I called for service ($350) and at that time it was an intermittent thing and the tech could not pinpoint the problem. Since the service call I have identified the above sequence to get the equipment to start.

I have changed the pressure switch and checked to make sure all condensation lines are clear.
Any ideas as to what is causing this unit not to operate correctly?
Thanks, diana

Sounds like you have something stuck on the control board. If you turn the power switch off wait 15 seconds and turn it back on will the vent motor start back up even if the t-stat is off??

Yes... when the furnace meets the t-stat set point it turns off but the vent motor (inducer fan) continues to run until I turn the power switch off. If I come back an hour later I can turn the power on and the inducer fan will start to run immediately, even if the t-stat is set below the room temp. usually the furnace will not start again due to the three flashes indicating a pressure switch error. I was able to get it going by very gently moving air in the tube to the pressure switch. Yesterday I replace the pressure switch and I am still having the same problem.

So, to recap, I can get the system to fire up, run, meet the set point, shut off - except for the inducer fan - shut off power, remove hose to pressure switch, turn power off and 50-50 chance it will run for one cycle... inducer fan will not turn off on its own.


Thanks for the reply... diana

If the inducer motor does not stop running the pressure switch will not open causing your pressure switch fault. Follow the wiring harness to the vent motor and see where it plugs into at the control board.

You can get it to run by pulling the hose off the pressure switch for a few seconds allowing the switch to open then put it back in place so that the pressure switch will make contact. As for the vent fan not stoping it has to be in the connection at the board or the board itself. Have you tried removing the stat and see if the motor will go off with the stat removed?

Thanks again for your troubleshooting... I am going back to the property tonight. I will see were the vent motor plugs into the control board then. I will check out the connection at the board. I am determined to fix the problem... I will also try pulling the t-stat and see how the motor reacts. Don't miss out on your thanksgiving dinner! More later...

I can see where the inducer motor plugs into the control board. The inducer motor runs when the system is powered. It does not turn off when the stat is removed...

What next?

Turn the power off to the furnace, remove the plug at the control board. Gently tap the control board with a screw driver plug the wire harness back in and try. If vent motor still runs, time for a new board

Thanks... I will try that tomorrow...

Do I tap anywhere on the board? Lets hope this fixes the problem..