Abstract: A locked cover for surrounding an air condition responsive controller such as a thermostat to prevent the setting from being altered by people unauthorized to do so. This is accomplished by baffles which are opposite the openings in the cover to prevent tools from being inserted through these openings to adjust the control. The baffles have openings through them to permit air to flow to the controller. In one form, the baffles are rigidly secured in position in an otherwise existing type of guard cover. In another form of the invention, there are a plurality of interlocking pieces which can be fitted together and placed within an existing cover to prevent access to the adjustment for the controller. In still another version, the guard cover is modified so that the openings for admission of air are not adjacent the controller, but are spaced outwardly therefrom with the openings through the baffles adjacent the controller.