This paper presents an accurate test campaign and investigation of module failures of a 2.65 kWp building integrated photovoltaic (BIPV) system installed in 2003 on a private roof in the Swiss Midlands. Due to a strong degradation of the plant’s power, the modules were dismounted and replaced within the guarantee. The PV modules were tested in SUPSI’s accredited Swiss PV Module Test Centre in accordance to IEC 61215 standards with visual inspection, insulation and wet leakage tests. Performance measurements at STC were done as well to verify their actual power. Electroluminescence (EL) and infrared images (IR) were taken to observe hotspots and cell damages. The authors observed significant differences of -10 to -75% to the nominal power of the modules after a 12-years operation period. Furthermore, laboratory tests confirmed measurement artifacts in I-V curves. The visual inspection showed damages and blistering of the back sheets, discoloration of PV cells and corrosion of the bus bars which can be related to the hotspots observed in the EL and IR- images.