They check and clean the nozzle. Vacuum the burn chamber to remove carbon deposits, same for the chimney base (that little metal hatch isn't really for Santa Claus). Inspect for leaks and burn-threw and calculate the efficiency level and compare that to previous checks.
You can also expect a visual inspection of the exteriors of the furnace and ignitor components.
Typically recommended as a twice a year task (spring and fall). Any fixes that are turned up are more convenient and cheaper to fix during a scheduled service call than 2am in the winter.
The newer, hyper efficient furnaces probably have a similar routine.
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