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. 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 checks the pump filter for socks, coins and hair ties, then tests the drain pump motor itself. 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
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. 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 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 tech tests the water inlet valve and its solenoid. 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 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 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.