Give it a little more time first. The oven may beep when the air reaches the set temperature, but the oven walls and racks can still be heating up. For foods that cook quickly, like cookies or pizza, wait another 10 to 15 minutes after the beep.
If the oven is only a few degrees off, you may be able to adjust the temperature yourself. Most ovens have a temperature offset setting. Check your owner’s manual for the correct steps.
If you have an electric oven, make sure the bake element glows red when heating. Look for breaks, blisters, or other damage. Also check the breaker. A tripped breaker or blown fuse can stop the oven from heating properly.
Still not reaching the right temperature? The temperature sensor, thermal fuse, wiring, or control board may be faulty. Book stove repair to have it tested.