Recently, a presence of so-called snail tracks has been reported in many photovoltaic (PV) fields. The influence of the phenomena on PV modules performance is ambiguous. In this paper, the authors have performed a series of tests on 20 polycrystalline modules, which have been operating in a moderate, intermediate climate for over 3 years, to evaluate the snail tracks impact. Besides commonly used techniques like visual inspection, electroluminescence, illuminated thermography and current-voltage characteristic determination, dark thermography was also applied. The analysis of uncertainty and the statistical approach were used to obtain reliable results. The visual inspection and electroluminescence tests confirmed in case of 96% of snail tracks that they are strongly correlated with micro cracks. In addition, a novel classification of snail tracks is proposed. There is no evidence that presence and number of snail tracks influence photovoltaic module power. Furthermore, illuminated and dark thermography revealed no clear relation between temperature field and snail tracks occurrence. (c) 2020 Elsevier Ltd. All rights reserved.