We had to put some money into our furnace last week... No fun!
We noticed we had hardly any heat, it would cut on and run and for a minute or 2 it was warm and then ran cold. Our furnace is a high end Trane model we put in about 8 years ago and winters here in Houston don't really put a big demand on it. Technician came out and tried changing out the sensor (kind of like a thermocouple) but that didn't fix it and the voltage from the circuit board was wrong which led him to suspect a bad board. He quoted $650 to replace the board. I looked on line and found some parts places would sell the board for $400 but it's a newer design so it has a new wiring harness and someone with the knowledge to transfer the 30 or so wires to the new board only made sense.
Not happy about the money spent but sometimes it's unavoidable.
The really frustrating thing is that we were told the unit had a 10 year warranty when we got it but later found out that it is a 5 year warranty unless the consumer goes to the trouble of registering it with Trane, then it gets a 10 year warranty. We used a different company to install the system from the repair company I use now so it's not their fault. But I don't remember the original installers telling us to register it, I think they either forgot or just failed to tell us because we would have done that for sure if we had know.