This is a weird rule of life: if something can be done by an ordinary person, but you hire someone else to do it, be prepared for a huge bill and/or chicanery.

This is a weird rule of life: if something can be done by an ordinary person, but you hire someone else to do it, be prepared for a huge bill and/or chicanery. Obviously not all HVAC is in the realm of a normal person's abilities, but much of it, and almost all problem diagnosis, only requires reading the diagram inside the furnace, using a multimeter, and a screwdriver or wrench. It's all very trouble-shootable. If you have those basic skills you should be able to identify what exactly is causing the issue, and the tech won't be able to lead you down the garden path.