Start with the rooms that changed
A single warm room may point to a closed damper, disconnected duct, or blocked register. A whole-home change is more likely to involve the filter, outdoor unit, thermostat, or equipment.
Check airflow first
Look at the return grille and filter, then confirm supply vents are open and not covered by furniture. A loaded filter can reduce airflow enough to cause poor cooling or ice on the coil.
Know when to call
Warm air from every register, short cycling, ice, water near the indoor unit, burning smells, and unusual electrical sounds are not good DIY projects. Turn the system off for active leaks or unsafe smells.
Keep a short symptom log
Write down the thermostat setting, room temperature, outdoor conditions, run time, sounds, and whether the problem changes after replacing the filter. A few observations can save time during diagnosis.
What a service visit should answer
Ask whether the problem is airflow, electrical, refrigerant, drainage, ductwork, or equipment condition. A good recommendation explains the finding, the risk of waiting, and the repair options.