Refrigerators and freezers
A fridge that runs but will not cool usually starts with the condenser coils. In city apartments those coils sit under the unit or behind a kick plate, packed with pet hair and dust that make the compressor work twice as hard. 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 checks the pump filter for socks, coins and hair ties, then tests the drain pump motor itself. 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. Loud banging during spin points to the suspension: worn shock absorbers, springs or a cracked spider arm on the drum.
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 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 sitting in the bottom of the tub starts with 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 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 split bake or broil element, and an oven that runs hot or cold usually has a drifting temperature sensor tested against the tech sheet resistance chart.
Microwaves
When the light and turntable work but the food stays cold, the magnetron, high-voltage diode or capacitor is a likely suspect. A microwave that is completely dead often has a blown fuse caused by a failed 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.