Long mild spells leave heating faults hidden until the first hard cold arrives.
Tye often goes through extended mild stretches when heating equipment sits unused. Then a strong cold front arrives and the system is asked to start, ignite, move air, and hold the indoor temperature without much warning. That first real demand is when an ignition problem, weak capacitor, failed blower component, dirty burner, safety-switch fault, or thermostat issue often becomes apparent.
A system can make heat without delivering enough of it to the rooms. A failed blower or a restricted filter stops proper airflow, leaving a furnace or air handler working while the house remains cold. Heating repair needs to confirm both sides of the job: that heat is being produced and that the system is circulating it through the house.
Combustion equipment also needs its venting and safety controls checked after a long off-season. Those parts are central to how the equipment starts and continues operating when colder weather finally calls for heat.