Non-professional repairs can have a negative influence on the deformation behaviour of a vehicle involved in a crash. The introduction by OEM’s of new materials and production techniques in cars makes it increasingly important that the repair of such cars is carried out with appropriate techniques and quality. These are the aims described in a project named “Fair Repair”, to which this paper is linked. This research project deals with the influence of non-professional repairs, on the behaviour of a car’s structure in an additional crash. KTI, with the support of the OEM (VW) tested the side structure of a VW Passat, MY 2005. With a side impact at 50 km/h (Euro NCAP standard) it was shown that a non-professional repair of a vehicle previously damaged in the same side impact scenario results in negative influences on the crashworthiness and protection afforded by the structure. The repair of the damage caused by the first crash was carried out using incorrect repair methods and equipment, e.g. welding machines. It is evident that the safety of such a vehicle after the non-professional repair is not to the same high level as that of the original build, or to the standard of a professionally repaired vehicle.