Often time people will think that their oven temperature has become inaccurate. This could be true, but the first thing you want to do is test to see if this is the case. Get a cheap oven thermostat from a local grocery store. Place the thermostat inside the oven directly in the center of the cavity. Turn the oven on to 350 degrees. When the preheat light goes off or the oven is done preheating the thermostat will not read correctly. The thermostat should always be lower than the 350 degrees you set the oven at because it weighted. Think of it as a piece of meat, the meat doesn’t automatically heat up to 200 degrees, it takes a roast several hours just to get to 140 degrees. The thermostat isn’t as big so it doesn’t take as long, but you should wait at least 30 minutes when testing the temperature to get an accurate reading. If it still doesn’t check correctly then you need to calibrate your oven thermostat. You could even have a faulty control board or a faulty thermostat and it will just need to be replaced. Most ovens can only be calibrated +/- 35 degrees so if its off by more than that, you have a faulty component.
This video explains how to remove an oven door on many higher end ovens, such as Kitchen Aid, Kenmore, Whilrpool,