This is usually a thermostat problem. When you set the thermostat at a set temperature and it does not reach that point it may be one of two problems: 
1. The thermostat may be defective.
2. The thermostat may be out of calibration. To check the calibration get a thermometer that you know is accurate. Put it in the oven and set the thermostat to 250º. Open the kick plate below the oven door and watch the burner flame, if it goes off before the oven reaches 250º you may be able to calibrate it.
To calibrate a thermostat remove the knob and check to see what type of thermostat you have. The thermostat is either a type with a round disk that has two screws holding it in place or it will have a D shaft with a small screw in the center of it. In either case only turn the disc or screw a fraction of a turn at a time and no more than a quarter turn either direction.
Continue to turn the disc or screw a fraction of a turn each time until you see the burner come back on. If you reach that quarter of a turn point and the burner does not come back on, the thermostat is defective and must be replaced. If the burner comes back on, watch the temperature of the oven and if it gets to within 5 or 10° of the preset temperature, you are good to go. It may take several tries to get it properly calibrated.
If you still can not get it calibrated within the temperature range, you need to replace the thermostat. All thermostats are preset from the factory and should not require calibration when installed. If you find that the new thermostat does require calibrating, follow the previous instructions.
When replacing your thermostat, remember: all the thermostats have a capillary tube with a bulb attached to the end of it. This is the part that senses the temperature in the oven. The bulb is attached to clips inside the oven. If the bulb is not put back in the same place, i.e. it’s just stuck in the oven cavity and left hanging, then the thermostat will run “wild,” meaning the oven cooks unevenly.
Remember to get that bulb back into those clips no matter how hard it may be.
There are two types of gas thermostats: BJWA and FDO type thermostats. BJWA thermostats are the most common type and older ones can be identified by the nickel sized hole on the front where the knob attaches. However, newer BJWA thermostats may not have that hole. FDO thermostats are usually found on pizza ovens and can be identified by the disc with numbers on it that sits behind the knob stem.
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