Without a proper seal, your oven can lose more than 20 percent of its heat. The result is that food takes longer to cook or cooks unevenly. To check the seal’s condition, open the oven door and locate the rubber or fiberglass gasket around the perimeter of the door. Feel for any broken, torn, or deformed areas, and close the door to see if you can find any leaks. If you do, replace the seal.