The ever‐increasing instalment capacity of Cu (In, Ga)(Se, S)2‐based photovoltaics calls for a better understanding and control of their reliability. In this paper, we show how using a coring‐based method, small samples can be extracted from full size commercial modules, and prepared for lab‐scale analysis. The method is applied to a Cu (In, Ga)(Se, S)2 (CIGS) module where a non‐reversible, propagating (‘wormlike’) defect has been created in a controlled partial shading experiment. Through current–voltage, photoluminescence and illuminated lock‐in thermography analyses on an undamaged part of the module, the method used is shown to yield fully functional, undamaged active cells, with a photovoltaic conversion efficiency above the full module efficiency. Where the wormlike defects were present, a typical strong shunting behaviour is observed, as well as an increased sulphur content near the edge of the wormtrails. Furthermore, the wormlike defect propagation is shown to be strongly influenced by the present of specific features near the interconnects, which could be the result of manufacturing. These results demonstrate the potential of coring to analyse module failure with all the laboratory tools available. They also shed some light on how wormlike defects, which are a rare but serious hazard for CIGS modules reliability, can form and propagate in commercial, monolithically interconnected modules.