1. Check the temperature, because most fridges are too warm
Start with a $6 fridge thermometer. Built-in displays report the temperature at the sensor, not on the shelf where your food sits, and they are commonly out by several degrees.
The target is 37 to 40°F, or 3 to 5°C, measured on the middle shelf with a thermometer left in place overnight. Milk and meat spoil roughly twice as fast for every 5°F above that range, so a fridge running at 46°F halves the life of everything inside it. The freezer should sit at 0°F or -18°C.
If the reading is high, check three things before assuming a fault. The door seal: close the door on a banknote and pull — if it slides out with no resistance, the gasket has hardened and should be cleaned with warm soapy water or replaced. The condenser coils at the back or beneath: a thick felt of dust makes the compressor work harder and cool less, and vacuuming them twice a year is a genuine efficiency gain. And the load: a fridge stuffed to the roof blocks the circulation vents at the back, which is why one overpacked shelf can leave the rest of the fridge warm.
Take a second reading in the door. The door shelves are the warmest part of any fridge, commonly running four to six degrees Fahrenheit above the middle shelf because they are exposed to room air every time someone opens it. That is why milk kept in the door sours days earlier than the same carton kept at the back, and why the moulded egg tray built into most door panels is the worst place in the appliance for eggs. Knowing that one number changes where you put things, which costs nothing at all.
Then leave room for air. Around 70 to 80 percent full is the efficient point for a fridge. A freezer is the opposite — it runs best nearly full, because frozen mass holds the cold.