In order to functionally test a leak detector (20) that comprises a partial pressure sensor (21), a testing device (30) having a variable volume (33) is connected to the leak detector. Said testing device can be a syringe. By changing the volume (33), the partial pressure of the test gas contained in the ambient air is changed. A test is carried out to check whether the leak detector (20) indicates the change. According to the invention, a vacuum pumping device having mechanical vacuum pumps is not required. The method is suitable for testing vacuum systems that contain a vacuum pumping device that is used in the leak test to generate a required vacuum. The service technician can perform the functional test of the leak detector (20) separately from the system and the vacuum pumping device.