The invention relates to a method for diagnosing the malfunctioning of a device for adding an additive into a fuel for a vehicle comprising an internal combustion engine, said method including: a step of analyzing the fuel in order to determine a variation in the amount of additive in the fuel; a step of comparing the variation in the amount of additive, measured during the previous step, with a theoretical variation in said amount; a step of sending information when the difference between the measured variation and the theoretical variation exceeds a set value. The invention also relates to a system for adding an additive into fuel and for diagnosis for a vehicle comprising an internal combustion engine, which is intended for implementing said method.