A fan can move air without the furnace producing heat. Controls, airflow, ignition, fuel supply and equipment faults can all be involved, so the symptom alone doesn't name the repair.
Before the routine service questions, two situations need a different response.
For a suspected gas leak, NV Energy says to leave the area and call 911 and 775-834-4100 from a safe location outside. Avoid anything that could spark near the leak.
If a carbon monoxide alarm sounds, CPSC directs people to get outside and call emergency services. Don't wait for a heating appointment to investigate an alarm.
Gas controls, ignition and safety devices need professional attention. Don't use instructions for another furnace to work on yours.
Tell the shop what the thermostat displays, whether you hear the system start, whether air comes from the vents and whether it stops again. Report a visible message exactly as shown without opening equipment to find one.
Include when the problem began, recent work and the type of system if you know it. If you're unsure whether you have a furnace, heat pump or combination, say so and send an accessible label photo.
These observations help the shop prepare. The technician still needs to check the actual equipment.
A replacement quote describes new equipment. It should also explain what was found in the existing system and why replacement is being recommended.
If repair is possible, ask what it fixes and what remains unresolved. If the technician identifies a safety problem, get clear instructions about using the equipment and follow them while considering your options.
Don't treat a second quote as permission to ignore a warning about unsafe equipment.
Keep the diagnosis beside the repair and replacement prices. Look for the parts and labor included in repair, and the equipment, connections, controls, permits and startup work included in replacement.
The repair or replace guide explains the comparison. If you still don't understand why the recommended work is needed, a second opinion can focus on that unanswered question.