Fuel injector comprising a nozzle assembly and a control valve assembly adapted to open or to close a spill orifice enabling variations of the pressure in the control chamber, the nozzle assembly comprising a nozzle body (62, 64) having a peripheral wall defining an internal bore (72, 78) in which a needle member extending from a head extremity to a pointy extremity is slidably guided and is adapted to translate under the influence of a differential of pressure between the pressure in a control chamber, generating a closing force on the head of the needle and, the pressure on the pointy extremity generating an opening force on said needle, the needle translating between a closed position, wherein a moving seating face integral to the pointy extremity of the needle is in sealing contact against a fixed seating face integral to the nozzle body thus closing a fluid communication and forbidding fuel injection via spray holes arranged through the peripheral wall of the nozzle body and, an open position wherein the moving seating face is lifted away from the fixed seating face thus opening said fluid communication and enabling fuel injection through the spray holes, the nozzle assembly being further provided with an electrical circuit comprising the needle member, the nozzle body, isolation means preventing electrical contact between the needle member and the nozzle body (62, 64) when the needle is in ballistic mode, between the open and the closed positions and, conductive means enabling electrical contact between the moving seating face and the fixed seating face when the needle is in closed position so that, an electrical signal enabling contact detection between the two seating faces (90, 92) is measurable between the needle member and the nozzle body (62, 64), characterized in that the nozzle assembly further comprises a piezoresistive device configured to continuously vary said electrical signal during the final closing displacements, or the initial opening displacements, of the needle, the variations of the signal being a function of the differential of pressure and, wherein said piezoresistive device is configured to continuously vary the electrical signal when the pressure in the control chamber rises up so said closing force becomes predominant over the opening force such that the needle moves and approaches the closed position, the electrical signal continuously varying from a closed level (R10-C) measureable when occurs the initial contact of the needle with the nozzle body, said fluid communication still being open, to a second level (R10-M) measureable when occurs the full closing of the needle sealing said fluid communication and, wherein said piezoresistive device is configured to continuously vary the electrical signal when the pressure in the control chamber drops down so the opening force becomes predominant over the closing force such that the needle in closed position initiates an opening displacement, the electrical signal continuously varying from said second level (R10-M) to an open level (R10-O) measureable when occurs the ultimate contact of the needle with the nozzle body said fluid communication being already open and the piezoresistive coating device is a piezoresistive coating film having thicknesses from 0.5μm to 2μm.