You can tell whether an oven door is losing heat by checking whether a piece of paper held in the closed door pulls out easily; if it slides out without resistance, the gasket is not doing its job. A hardened or torn gasket, heat leaking around the edge of the glass, a kitchen that gets hotter than expected, and recipes that consistently take longer to cook are other signs of the same problem.
The door gasket is a strip of silicone or fiberglass that keeps the oven’s hot air inside. Over the years, heat and grease can make it hard, cause it to shrink, or break it in places. When the gasket stops working, the oven continually loses heat. The thermostat cycles on more often to compensate, and even when the oven reaches its target temperature, the inside cannot maintain a stable environment. As a result, the area near the door stays noticeably cooler and cooking becomes uneven.
The simplest check is the paper test. With the oven cold, place a bill or a thin strip of paper between the door and the oven, then close the door. If the paper pulls out without resistance, the gasket is not sealing tightly at that spot. Repeat the test separately along all four edges of the door and at the corners, because a gasket often fails at just one point. You can use a thin paper napkin instead of regular paper. If the door does not close fully, shifts, or needs to be pressed shut, check the hinges too.
You can also look for signs while the oven is running. You may feel hot air coming from the edges of the door; check by moving your hand a few centimeters away from the edge, without touching the glass or metal. A leak may be present if the outer glass gets hotter than usual, the kitchen gets hotter than expected, or condensation continually builds up and drips inside the oven glass. Condensation on the inner glass during the first few minutes is normal; if it continues throughout cooking, that means something different. Keep the check brief while the oven is running.
The effects become noticeable in the kitchen. Recipes in an oven that loses heat take longer than their stated cooking times even though the display shows the correct temperature; cakes do not rise, the crust forms late, and the difference between the upper and lower racks grows. These signs can be confused with a faulty thermostat. To tell the difference, hang an oven thermometer inside: if it shows the target temperature but cooking still takes longer, the gasket is likely the problem. The less often you open the oven door during cooking, the less pronounced the difference will be.
On most models, users can replace the gasket themselves. Note the oven’s model number and buy a replacement part to match; the gasket is usually installed as a strip that fits into a channel. Before replacing it, clean the gasket channel and the edge of the door with warm, soapy water, because built-up grease can prevent the gasket from seating properly. Repeat the paper test after installing the new gasket. If a hinge is loose or the door is bent, leave the repair to a service technician. A new gasket may give off a slight odor the first time you cook.