Precise determination of the technical condition of agricultural machinery affects the quality of technical service and its costs. Diagnostics based only on the observed classical phenomena or measurements made is burdened with considerable error. The paper presents a method of diagnosing based on logical analysis of symptoms or damage, using technical knowledge. Verification of methods was carried out by diagnosing a hydraulic lift of an agricultural tractor, when it was observed that it does not maintain predetermined heights. The presented method is universal and can be used for diagnostics of other units, however, it requires technical knowledge on the possible inefficiencies and the symptoms caused.