Expect questions about your fuel, the age of the equipment, and what happened just before it stopped. On an oil system, a technician typically checks the fuel supply, filter, nozzle, and ignition and may bleed air from the line after a run-out. On propane or gas, they test the ignition, flame sensor, and venting with instruments rather than guesswork.
Oil burner work calls for a technician with a Department of Fire Services certificate, and gas or propane work for a Massachusetts gas fitter. If the visit turns into a replacement, Carver's Building Department takes gas, plumbing, and electrical permits on paper, so a new system usually won't go in on the night of the call.
A careful contractor will tell you whether the fix is temporary, what part failed, and whether the equipment is safe to run until a fuller repair. Ask them to write that down before they leave.