It is not uncommon for an oven to be off by as much as 25 to 50 degrees, which can greatly affect the results of your cooking. To test the internal temperature of your oven, place an oven-safe thermometer into the middle of the oven. Preheat the oven to the desired temperature. Once the oven has properly preheated, check the temperature on the thermometer. If the temperature does not match the temperature set on the dial, you’ll need to adjust the dial accordingly until the internal thermometer reaches the desired temperature. Until you really understand how your oven works, continue to monitor the oven thermometer, rather than relying on the external dial of your oven. You can also contact the manufacturer to have your oven calibrated properly.