Arc voltage fluctuations of a plasma spray torch are primarily an indication of the movement of the arc attachment inside the anode nozzle. These fluctuation have been shown to influence the deposition process. In order to detect changes in the operating conditions which affect coating quality, a method has been developed for on-line analysis of these fluctuations. Voltage fluctuation have been recorded together with light emission fluctuations and with acoustic emissions from the plasma jet and analyzed on-line using a workstation operating with the LabView environment. Anodes with different wear characteristics have been examined in this study. A clear correlation has been found between the changes in the dominant frequencies of all three signals and the conditions of the torch anode and the coating properties. Appearance of a group of frequency peaks in the 2 to 5 kHz range indicates a more unstable plasma jet and is correlated with anode erosion and increased coating porosity. The results of this study provide us with a convenient method to detect coating deterioration due to anode erosion.