The main purpose of diagnosing the condition of the electrostatic precipitator should be to present a series of future operating activities that make it possible to predict unfavourable situations in the plant and reduce the risk of damage, which ultimately lead to the plant being excluded from further operation. Maintaining the reliability of power units is important for the correct operation of the national power system in Poland. Power system failures generate serious effects in the form of a lack of access to electricity in the area covered by the failure. Diagnosing the condition of power units should enable verification of the degradation process or also identifying damage caused by non-stationary operation of plants. The scope and methods of diagnosing technical condition of power units should be based, among other things, on the analysis of results of work conditions and unit failure. Determining the limits of electrostatic precipitator fitness status should be based on two main issues. Designation of the relationship between the occurrence of a symptom and a change in the technical condition of the machine and determining the limit values of measured symptoms.