Furnace starts out great & then later not?

Blanco1 wrote:
Where would I find the converter & how would I check it?

Converter is generally in close proximity to the main fuse and breaker panel.
If you have about 13.6 volts at the fuses the converter is fine.

Now you need to remove the furnace for inspection. If you are not comfortable with that you will need to go in for service.

There is no external knob or switch that is going to resolve the furnace trouble.