The article presents one of the techniques of analytic geometry, referred to as the box method or the Minimum Bounding Rectangle (MBR) approach, which was applied in the diagnostics of a common rail piezoelectric injector. By extending the standard test procedure with a computational phase, it was possible to estimate the fuel dosing surface areas and compare them with reference values. It was demonstrated that the proposed method is particularly useful in situations where a clear assessment of the injector’s technical condition is difficult to obtain. This approach eliminates the need for additional measurements during the active testing phase, without increasing the final costs or the time required for the regeneration process.