help: a two-stage gas furnace problem

I have a two-stage gas furnace with a one-stage thermostat running perfectly since 1998.

For about two weeks ago, it started to have the problem as described below.

The furnace may stop generating heated air when the target temperature is more than 2 degrees higher than the current room temperature, which usually occurs after running for about 10 plus minutes, while the fan will keep blowing cold air continuously.

The heater was running with the "auto" and "heat" mode. The furnace is a Trane XL 80 and the thermostat is TX1500E Programmable.

When it stopped working, the light indicator became steady red, which signals to "replace the circuit board" according to the print on the circuit board (White-Rogers 50A51).

However, I can still use the furnace by gradually adjusting the temperature with the following steps.

- set the "auto" plus "heat" mode, and start a temperature at 59
- it will run normally and shut itself off after reaching the set temperature
- a few minutes afterwards, increase the temperature by 2 degrees
- again, the heater starts running normally
- repeat the last 2 steps

Again, if I set the temperature 3 or more degrees higher than the current room temperature, the furnace will stop working before reaching the desired temperature; and the only exception is when it starts at the 59 degrees, which will work normally even when the room temperature is more than 2 degrees lower.

I have one service call for $80, and the person suggested to replace it with a new furnace which costs from $2,100 to $3,700, depending on the efficiency level of the new equipment.

Currently, I'm waiting for a second service call for $120/hour from a Trane recommended professional, and I start wondering whether there is a more cost effective solution.

Even though I could live with the dis-functional heater, the observation above seems to suggest that the problem could be a simple one, probably a re-adjustment of the thermostat, or a re-configuration (re-wiring) of the control circuit board, or a replacement of a two-stage thermostat.

Thus, I hope someone in this community can help me.

Thanks,
--- Chang

If the code says replace circuit board, look up the part # online and buy one. Probably less than the 120$ call.

Good suggestion.

Do I need the exact same old model, or a similar new model works just fine?

For example, I see several White-Rogers 50A51-***, whose last 3 digits do not match mine. Do you know how different they are?

Thanks,
--- Chang

Order that part online and on the phone, from a dedicated hvac or appliance part with a helpline. Those last three digits should be evaluated. The MFG. should have a parts line also at least to get you the right part#.

It can take a bit of work to sort it out, maybe why the HVAC guys charge so much.

I agree with ballvalve. Get your serial number off the unit along with the part number of your board and the Trane store in your area should be able to help.
You may be able to look it up on www.trane.com

Thank you all for the suggestion.
I will follow it on and report back.