Refrigerators and freezers
When a refrigerator runs without cooling, the condenser coils are usually the first stop. 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. Heavy frost on the evaporator coil points to the defrost system, so the heater, defrost thermostat and control board each get a continuity check with a multimeter. A clicking sound every few minutes with no hum often means a failed start relay or overload on the compressor.
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. A machine that refuses to start or spin often has a failed lid switch on a top-loader or a bad door lock assembly on a front-loader. Heavy banging on spin points to suspension trouble: tired shocks, worn springs or a cracked drum spider.
Dryers
An electric dryer with no heat usually has a blown thermal fuse or dead heating element, while a gas dryer with no heat often has a weak igniter or bad valve coils. A drum that will not turn points to a snapped belt, and a squeal or thump points to worn rollers or an idler pulley. What people tend to miss is that a thermal fuse fails for a reason, and the reason is usually a blocked 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 water inlet valve and its solenoid get tested. Dishes coming out wet and cold suggest a failed heating element or high-limit thermostat.
Ranges, ovens and cooktops
When a gas oven clicks without lighting, the hot surface igniter has usually weakened and no longer draws enough current to open the safety valve. The tech confirms that with a clamp meter and may check gas pressure at the regulator with a manometer. 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
When the light and turntable work but the food stays cold, the magnetron, high-voltage diode or capacitor is a likely suspect. A totally dead microwave often has a blown internal fuse triggered by a bad door switch. Never open a microwave yourself: the high-voltage capacitor can hold a lethal charge long after the unit is unplugged, and a trained tech discharges it safely before touching anything.