Oil furnace issues can stem from various sources, including clogged filters, malfunctioning burners, and defective thermostats. Regular maintenance can help prevent these issues, but addressing them promptly is crucial to maintain efficient heating and avoid potential safety hazards. If the furnace fails to ignite, check the oil filter and replace it if necessary. A faulty burner may require cleaning or replacement. Troubleshooting and repairs should be performed by a qualified technician for safety and effectiveness.