A fixing element fixes a part to a support wall. When the fixing element is inserted into an opening in the support wall and reaches a predetermined end position, the fixing element locks to the support wall. The fixing element includes a stop element, a snap-on element, and an unlocking element. The stop element bears against a side of the support wall when the fixing element is locked to the support wall. The snap-on element is positioned on an inner side of the support wall when the fixing element is locked to the support wall. The unlocking element is connected to the snap-on element. The unlocking element is configured to permit the snap-on element to be unlocked and the fixing element to be removed from the opening. An installation tool and a testing tool may be used in conjunction with the fixing element.