To diagnose evaporator fan issues, start by checking the air filter and ensuring it is clean. Next, inspect the fan motor and belts for signs of wear or damage. Listen for unusual noises, such as grinding or squeaking, and check for reduced airflow from vents. If the issue persists, use a multimeter to test the fan motor’s electrical connections and voltage supply. Consult a professional HVAC technician if you are unsure or uncomfortable with the diagnosis process.