Gas oven thermostats control the oven temperature by opening and closing a switch in relation to the interior temperature of the oven. Thermostats allow you to set your oven at an exact temperature for cooking food, and need to be recalibrated if testing shows they are more than -3.89 degrees C off the specified temperature. Calibrating the oven will vary with the model of the oven, but can be done properly as long as you follow the instructions in the manual that came with your oven. Some ovens do not have a calibration ring, and in such cases the thermostat must be removed and replaced.
Open up the panel where the thermostat is. This will vary according to the type of gas oven you have. Some ovens have the panel in the back; others have doors that need to be unscrewed. The instructions manual will tell you where the thermostat is located.
Pull out the thermostat knob carefully. In some cases it may need to be screwed off of the thermostat.
Locate the calibration ring on the back of the thermostat and adjust the ring to the desired specifications. Again, consult your instructions manual, which will give you specific details about your particular type of gas oven.
Detach the capillary tube if there is no calibration ring for your oven. Slide it through the back of the oven extremely carefully, as the tube wiring is flammable and breaking it could be disastrous.
Unscrew the thermostat and gently pull it out from the oven. You do not need to remove the wires.
Slide the new thermostat in and screw it in place. Reattach the capillary tube, again being very careful when handling it.