Your said setting is 21 and it shuts off at 19. Does it happen each time after you reset?
If it does shuts off at 19 every time, I Suggest you to do a simple test. change the setting up to 23, make a note of the temperature reading when it shuted off, if it shut off at 19 again, it most likely the over temperature thermostat protection switch beside the burner compartment is not working properly,(Burner compartemant is too hot before the room temperature reachs to the setting) if it shut off at 23 or higher, the temperature controller may have problem, remove battery to reset the controller, if you can also remove all the wires that connected to the controller. Good Luck!
If it does shuts off at 19 every time, I Suggest you to do a simple test. change the setting up to 23, make a note of the temperature reading when it shuted off, if it shut off at 19 again, it most likely the over temperature thermostat protection switch beside the burner compartment is not working properly,(Burner compartemant is too hot before the room temperature reachs to the setting) if it shut off at 23 or higher, the temperature controller may have problem, remove battery to reset the controller, if you can also remove all the wires that connected to the controller. Good Luck!