故障按在線顯現情況，可分為實際和潛在兩種。實際故障又稱為功能性故障，由于這種故障的實際存在，使液壓原統不能工作或工作能力顯著降低，如關鍵液壓元件被損壞等；潛在故障與緩發性故障相似，尚未在功能性方面表現出來，但可以通過觀察及儀器測試出來它的潛在程度。

        故障按發生的原因，可分為人為的和自然的兩種。由于設計、制造、運行安裝、使用及維修

不當等造成的故障均稱為人為的故障；由于不可抗拒的自然因素(如磨損、腐蝕、老化及環境等因素)產生的故障均屬于自然故障范圍。

        液壓設備的故障，一般在初期因設計、制造、運輸、安裝、調試等原因而故障率較高，隨著運用時間延長及故障的不斷排除，故障率將逐漸降低。到了設備使用后期，由于長期使用過程中的磨損、腐蝕、老化、疲勞等而逐漸使故障增多。只有在使用中期，設備故障才趨于隨機的較穩定期，也就是設備的有效工作壽命期。但如果由使用不當或對潛在的故障不及時診斷與排除，即使在有效壽命期也不能排除出現突發性的各種嚴重故障。故認真研究和總結各設備故障及診斷技術，則是正確及時地排除各種故障的前提。

Faults can be classified into actual and potential ones according to their online appearance. Actual fault is also known as functional fault, because of the actual existence of this fault, the original hydraulic system can not work or work ability is significantly reduced, such as the key hydraulic components are damaged; A potential fault, similar to a delayed fault, has not been demonstrated in terms of functionality, but its potential can be measured by observation and instrumentation.

Failure according to the cause of occurrence, can be divided into artificial and natural two. Due to design, manufacture, operation installation, use and maintenance

Fault caused by improper fault are called artificial fault; Due to irresistible natural factors (such as wear, corrosion, aging and environmental factors) caused by the fault are natural fault scope.

The failure rate of hydraulic equipment is generally higher in the initial stage due to design, manufacturing, transportation, installation, debugging and other reasons. With the extension of the operation time and the continuous elimination of the fault, the failure rate will gradually reduce. In the late stage of equipment use, due to long-term use of wear and tear, corrosion, aging, fatigue and so on and gradually make the fault increase. Only in the middle period of use, the equipment failure tends to random stable period, that is, the effective working life of the equipment. However, if the potential fault is not diagnosed and eliminated in time due to improper use or failure, it is not possible to eliminate all kinds of sudden serious faults even in the effective life. Therefore, it is the premise of correct and timely troubleshooting to study and summarize the equipment faults and diagnostic techniques.