The most common causes of furnace breakdowns include lack of maintenance, dirty or clogged filters, ignition or pilot control issues, thermostat malfunctions, wear and tear on components like belts and bearings, and electrical system failures. Ignoring regular maintenance can exacerbate these issues, leading to breakdowns when the furnace is under heavy use. Prompt attention to maintenance tasks and addressing any emerging issues can prevent most furnace breakdowns, ensuring reliable heating performance during colder months.