What collects above the cooktop
Cooking vapour carries fine grease and moisture upward. Over time, residue can coat filters, vents, cabinet surfaces and parts around an over-the-range microwave. Heavy buildup can reduce airflow and contribute to sticky latches or controls.
Simple prevention
Use the exhaust fan while cooking, keep the manufacturer-required clearances, and clean or replace filters on the schedule specified for the appliance. Avoid spraying cleaner directly into vents or controls.
Safety note: Appliance symptoms can have more than one cause. Microwaves contain high-voltage components that may retain a dangerous charge after unplugging. Do not open the cabinet without proper training.