This paper reports the results of an analysis of indicators describing the effectiveness of actions taken and repairs made by the maintenance services in a food industry company which had implemented a new manufacturing execution system (MES) 10 months prior to the study. The application of the above effectiveness indicators plays a significant role in the rationalization of functioning of maintenance services. Therefore, it is vital that they are calculated correctly and interpreted in a way that has a positive effect on the organization of maintenance works. The paper investigates four effectiveness indicators employed by the maintenance services of the company in question, i.e., mean time to failure (MTTF), mean time between failures (MTBF), mean time to repair (MTTR) and overall equipment effectiveness (OEE). The objective of the analysis was to verify the correctness of determination of the above indicators in the analysed company. In addition, the study was to determine whether the use of correctly determined indicators and results interpretation could lead to a higher effectiveness of the actions taken by the maintenance services department. Moreover, the paper presents a diagnosis of problems connected with incorrect determination and visualization of the above-mentioned indicators in the analysed company.