A first test injection of a fuel injector is performed with a first test Energizing Time and the actual fuel quantity injected is measured to obtain a first point of a characteristic curve representing a relationship between the injector Energizing Time and the fuel quantity which is actually injected. A ratio between the actual fuel quantity injected and a nominal fuel quantity corresponding to the first test Energizing Time is calculated and used to determine a plurality of corrected Energizing Times for subsequent test injections. The subsequent test injections are performed using the corrected Energizing Times and the actual fuel quantity that has been injected during each one of the subsequent test injections is measured to obtain further points of the characteristic curve. The characteristic curve of the fuel injector is modeled based on the obtained further points and used to control the operation of the fuel injector.