Cleaning is part of the work
Coils, burners, drains, filters and other components need appropriate cleaning based on the equipment and season. “Maintenance” should not be only a visual inspection.
Performance should be checked
A technician should evaluate how the system is operating, not simply whether it turns on. Airflow, temperature, electrical operation, refrigerant performance, combustion and controls may all matter depending on the system.
Small changes are useful information
A part does not have to be failed to show deterioration. Ongoing maintenance gives the technician multiple points of comparison over time.
Timing matters
Maintenance before the heating and cooling seasons creates a better chance to address problems before extreme weather increases both system load and service demand.
Service history compounds
The longer the same service organization knows the equipment, the less every future call has to start from zero.
Talk to a technician when the answer depends on your system.
General guidance can help frame the decision, but actual repair, maintenance and replacement recommendations should follow inspection of the equipment and the home.