Do it before the system is struggling
A tune-up is useful when the system is still running but you want a better read on coil condition, electrical basics, airflow, readings, and drain concerns.
Homeowner Guide
When to schedule an AC tune-up.
The best time is before a long heat stretch, when scheduling is easier and the system has not been pushed hard yet.
When to choose a diagnostic instead
If the house is not cooling, the outdoor unit will not start, water is showing up, or the system is short cycling, request the diagnostic visit instead of treating it like routine maintenance.