Refrigerators and freezers
A fridge that runs but will not cool usually starts with the condenser coils. In apartments, those coils hide underneath or behind the toe grille, matted with dust and pet hair that force the compressor to overwork. Then the tech listens for the evaporator fan behind the freezer wall, since a failed fan keeps the freezer cold while the fridge side warms up. If frost has built up on the evaporator coil, the defrost system is next: heater, defrost thermostat and control board, each checked for continuity with a multimeter. Repeated clicking with no compressor hum often means the start relay or overload has failed.
Washers
A washer full of standing water almost always comes down to the drain path. The tech clears the pump filter of coins, socks and hair ties, then tests the drain pump motor directly. If the machine will not start or spin, the lid switch on a top-loader or the door lock assembly on a front-loader is a common culprit. Heavy banging on spin points to suspension trouble: tired shocks, worn springs or a cracked drum spider.
Dryers
No heat in an electric dryer usually means a blown thermal fuse or a burned-out heating element, and no heat in a gas dryer often means a weak igniter or failed gas valve coils. A drum that sits still suggests a broken belt, while squealing or thumping points to worn drum rollers or the idler pulley. Here is the part people skip: a thermal fuse blows for a reason, and that reason is usually a clogged vent. In high-rises with long shared vent runs, the tech will check airflow at the exhaust before calling the job done.
Dishwashers
Water pooling in the tub starts with a check of the filter and drain hose, then the drain pump. If the unit hums but never fills, the tech tests the water inlet valve and its solenoid. Dishes that come out cold and wet usually mean a failed heating element or high-limit thermostat.
Ranges, ovens and cooktops
A gas oven that clicks but will not light usually has a weak hot surface igniter that no longer pulls enough current to open the safety valve. The tech checks that with a clamp meter, and may use a manometer to confirm gas pressure at the regulator. Electric ovens that heat unevenly often have a broken bake or broil element, and ovens that run hot or cold usually have a drifting temperature sensor, checked against the resistance chart on the tech sheet.
Microwaves
If the light and turntable work but food stays cold, the magnetron, high-voltage diode or capacitor may be at fault. A totally dead microwave often has a blown internal fuse triggered by a bad door switch. Do not open a microwave on your own, because the high-voltage capacitor can hold a lethal charge long after unplugging, and a trained tech discharges it safely before any work begins.