Manually handled tool for installing and servicing detectors, which are attached or are being attached to a fixed subsurface, which are designed for monitoring ambient conditions and to each of which a detector identifier is assigned, comprising - a base carrier having an fixing appliance, by means of which the tool can be mechanically connected to a rod so as to guide the tool attached to the rod from a user holding the rod to a mounting point for a detector on a fixed subsurface or on a detector mounted on a fixed subsurface, respectively, - a receiving appliance on the base carrier for attaching a reading device for determining the detector identifier of a mounted detector, and, - in the base carrier, a receiving unit having an upper receiver, facing the fixed subsurface or the detector mounted on the subsurface, respectively, and a lower receiver, facing away from the fixed subsurface or the detector mounted on the subsurface, respectively, wherein - the upper and the lower receiver (14, 15) are designed in such a way that a functional unit, designed for installing or servicing detectors, can be manually inserted into each of them and can be removed again for replacement with another functional unit, and - wherein the functional units, which can be manually inserted and removed again for replacement with another functional unit, are designed as the following functional units: - a mounting unit for inserting a detector insert into a detector socket, mounted on a subsurface, by means of the tool, or for removing a detector insert from a detector socket, mounted on a subsurface, by means of the tool, the upper receiver being designed in such a way that a detector insert inserted into it is mechanically connectable to or detachable from an associated detector socket, mounted on a fixed subsurface, by rotating the tool by means of the rod, and the tool having a replaceable adapter, which can be inserted into the upper receiver and removed again, for adapting the upper receiver to the shape of a specific detector or detector insert, and - a testing unit for testing the functionality of mounted detectors by means of the tool. - a base carrier having an fixing appliance, by means of which the tool can be mechanically connected to a rod so as to guide the tool attached to the rod from a user holding the rod to a mounting point for a detector on a fixed subsurface or on a detector mounted on a fixed subsurface, respectively, - a receiving appliance on the base carrier for attaching a reading device for determining the detector identifier of a mounted detector, and, - in the base carrier, a receiving unit having an upper receiver, facing the fixed subsurface or the detector mounted on the subsurface, respectively, and a lower receiver, facing away from the fixed subsurface or the detector mounted on the subsurface, respectively, wherein - the upper and the lower receiver (14, 15) are designed in such a way that a functional unit, designed for installing or servicing detectors, can be manually inserted into each of them and can be removed again for replacement with another functional unit, and - wherein the functional units, which can be manually inserted and removed again for replacement with another functional unit, are designed as the following functional units: - a mounting unit for inserting a detector insert into a detector socket, mounted on a subsurface, by means of the tool, or for removing a detector insert from a detector socket, mounted on a subsurface, by means of the tool, the upper receiver being designed in such a way that a detector insert inserted into it is mechanically connectable to or detachable from an associated detector socket, mounted on a fixed subsurface, by rotating the tool by means of the rod, and the tool having a replaceable adapter, which can be inserted into the upper receiver and removed again, for adapting the upper receiver to the shape of a specific detector or detector insert, and - a testing unit for testing the functionality of mounted detectors by means of the tool.