Unplug the oven from the wall. This is necessary to prevent electrical shock. The voltage may be fatal.
Open the door of your oven.
Look for the oven sensing bulb. It is normally located on the back wall of the oven interior.
Twist the bulb slightly with your fingers to make sure it is not loose. If it feels loose, tighten it by turning it counterclockwise. It may have been dislodged due to vibrations from the door opening and closing and may even be on the floor of the oven. If so, put it back where it belongs.
Plug the oven back into the wall outlet.
Checking the Temperature
Set the oven thermometer inside the oven. This type of thermometer typically has a hanger attached to it, allowing you to hang it from the rack in the center of the oven. Read the directions on the thermometer before use to determine the best placement.
Preheat the oven. Setting it to a temperature of at least 325 degrees Fahrenheit is necessary.
Wait for the preheat indicator to go off.
Open the door of the oven and check the temperature on the thermometer. It should match the oven setting. Repeat this step two or three times on different days to see if the difference in temperature is consistent. If it is 25 degrees less or more than the setting, in the future adjust your baking temperature to accommodate this. For instance, if the oven thermometer states the temperature is 400 degrees Fahrenheit and you set the dial to 375 degrees, subtract 25 degrees from the temperature needed when using the oven.
Call a repair person to replace the thermostat if you have trouble remembering to adjust the temperature or don't want to be bothered with doing it. If the temperature varies more than 25 degrees Fahrenheit or the difference is not consistent, it is likely time to replace the thermostat.