Common furnace repair problems often vary depending on the age and model of the furnace, but several issues frequently arise:
1) Clogged Filters: One of the most common issues is clogged air filters, which can restrict airflow and reduce heating efficiency. Regularly replacing or cleaning filters can prevent this problem.
https://www.quora.com/What-are-common-furnace-repair-problems/answer/Next-Service-AC-And-Heat