The size of the loose patch names the fault — the depth of the leak does not
Before the test, one fact about how the seal works, because it explains everything else. A fridge gasket is not held shut by the latch. It is pulled onto a steel frame by a magnet strip moulded inside the rubber. The magnetic force is small — tens of grams per centimetre — and it has to act through two soft air bulbs that are there to take up the unevenness of the frame. That means the seal is a system with almost no margin: lift the rubber two millimetres anywhere and the magnet cannot close the gap, because the force falls off very quickly with distance.
That is why the paper test works, and why it is a better instrument than your eyes. Trap a strip of paper in the gap and pull it out: resistance means the magnet is gripping, an easy slide means it is not. Walk it round the whole door at six or eight points — top, bottom, and two or three down each side. A torch shining inside a dark kitchen with the door shut will show the same thing, and it is worth doing both, because the paper tells you about grip and the torch tells you about light.
Now the part that turns a test into a diagnosis: read the shape of the loose region, not its severity. The three possible causes leave geometrically different footprints. A single spot is something physical lifting the rubber — a crumb, a dried drip, a squashed corner after a move. A whole edge, in a diagonal relationship with the opposite edge, is a hinge that has dropped, because the door pivots on that axis and a dropped pivot turns a rectangle into a parallelogram. A whole ring is the rubber itself, gone hard. Same test, three different answers, and the amount of daylight at any one point tells you none of it.
Three faults, three answers
One or two points slide easily is the cheapest and the most common. Sugar syrup, juice or gravy dries in a fold of the gasket and holds the rubber a couple of millimetres off the frame, which is already more than the magnet can span. Clean it properly: warm water with a little washing-up liquid, and a cotton bud worked along the inside of each fold rather than a cloth over the outside, because the fold is where the residue sits. Then dry it, and retest. If the rubber has been left squashed — a new fridge that spent a week strapped shut, or a door propped open against something — warm it with a hairdryer at ten centimetres, moving constantly, for about thirty seconds, press it back into shape with your fingers and hold it while it cools.
A whole edge slides and the opposite edge grips is the hinge, and this is the one people miss by buying a gasket. A loaded fridge door is heavy, and the hinge pin slowly drops. The door then hangs as a parallelogram rather than a rectangle, so the gap opens along one edge and closes along the other. Look along the door with the fridge shut: the gap between the gasket and the frame is wider at one end of the edge than the other. Adjusting the hinge is free — most top hinges have two screws you can slacken, lift the door and retighten, and some have shims. If a new gasket leaks and the gap is still wider at one end, it was never the gasket.
The whole ring slides easily is the rubber, and here you do buy a part. The signature is in your fingers: a healthy gasket is soft and springs back, an aged one feels like hard rubber, does not return, and may even rustle faintly as the surface has begun to craze. Order it by model number, because gaskets are moulded per model and are not interchangeable between batches. Soak the new one in warm water to soften it before fitting, and warm the corners with a hairdryer afterwards, then run the paper test all the way round — a new gasket that is not seated at the corners leaks exactly like the old one.