You can also test whether the sensor is working properly by using an ohmmeter. As the temperature inside the oven rises, the resistance reading of the temperature sensor should also rise.

If it's still not heating correctly, you can use an ohmmeter to test that the sensor is working. The resistance of the sensor should rise with the internal temperature of the oven.

Get an oven thermometer, learn the quirks of your appliance and be willing to call in the the pros when there are signs of trouble.