Reproducibility of plasma spray process and resulting deposits is one of the most essential requirements for its application in industrial environment. In most cases, only controlling process input parameters do not suffice for achieving stability and reproducibility of the process. Besides short time period plasma instabilities such as arc root fluctuation, process deviations may arise in longer time operations due to electrodes wear which can be identified by on-line diagnostics of the plasma spraying process.In this study, two on line diagnostic systems were employed to investigate long-term behaviour of a standard F4 plasma gun with a deLaval-nozzle. The Plasma Instability Analysis (PISA) was applied to identify electrode wear and plasma fluctuation phenomena within the gun whereas Particle Flux Imaging (PFI) was used to correlate the electrode wear with the plasma jet characteristics at the nozzle exit. The long-term performance of electrodes was investigated under continuous operation or under periodic re-ignition of the gun. A good correlation between aberrations in the plasma process observed with the PFI and changes in the frequency spectra detected by the PISA was found.