Method for diagnosing the state of operation of at least three groups of at least one fuel injector (15A, 15B, 15C, 15D) which are designed respectively to inject fuel into a corresponding number of cylinders (16A, 16B, 16C, 16D) of an internal combustion engine, the said internal combustion engine comprising an injection line comprising a fuel pump connected to the said groups of injectors (15A, 15B, 15C, 15D) via a fuel distribution rail, the said method being characterized in that it comprises: - a first operation which is implemented for each cylinder (16A, 16B, 16C, 16D) concerned and which comprises steps consisting in: a) acquiring a measured value (P 1,m) of a first pressure in the said distribution rail, b) injecting fuel into the cylinder (16A, 16B, 16C, 16D) concerned using the corresponding group of injectors (15A, 15B, 15C, 15D) according to a determined fuel quantity setpoint (Q inj,A, Q inj,B, Q inj,C, Q inj,D,), c) estimating an estimated value (P 2,e) of a second pressure in the said distribution rail at the end of step b), as a function of the said measured value (P 1,m) of the first pressure, d) acquiring a measured value (P 2,m) of the said second pressure, e) comparing the said estimated value (P 2,e) with the said measured value (P 2,m) of the second pressure, and deducing an error factor (α inj,A, α inj,B, α inj,C, α inj,D) associated with the said cylinder (16A, 16B, 16C, 16D) concerned as a function of the difference between the estimated value (P 2,e) and the measured value (P 2,m) of the said second pressure and of the said fuel quantity setpoints (Q inj,A, Q inj,B, Q inj,C, Q inj,D) and - a second operation of comparing the error factors (α inj,A, α inj,B, α inj,C, α inj,D) associated with the said cylinders (16A, 16B, 16C, 16D) and detecting a potential malfunctioning of at least one of the said groups of injectors (15A, 15B, 15C, 15D). - a first operation which is implemented for each cylinder (16A, 16B, 16C, 16D) concerned and which comprises steps consisting in: a) acquiring a measured value (P 1,m) of a first pressure in the said distribution rail, b) injecting fuel into the cylinder (16A, 16B, 16C, 16D) concerned using the corresponding group of injectors (15A, 15B, 15C, 15D) according to a determined fuel quantity setpoint (Q inj,A, Q inj,B, Q inj,C, Q inj,D,), c) estimating an estimated value (P 2,e) of a second pressure in the said distribution rail at the end of step b), as a function of the said measured value (P 1,m) of the first pressure, d) acquiring a measured value (P 2,m) of the said second pressure, e) comparing the said estimated value (P 2,e) with the said measured value (P 2,m) of the second pressure, and deducing an error factor (α inj,A, α inj,B, α inj,C, α inj,D) associated with the said cylinder (16A, 16B, 16C, 16D) concerned as a function of the difference between the estimated value (P 2,e) and the measured value (P 2,m) of the said second pressure and of the said fuel quantity setpoints (Q inj,A, Q inj,B, Q inj,C, Q inj,D) and a) acquiring a measured value (P 1,m) of a first pressure in the said distribution rail, b) injecting fuel into the cylinder (16A, 16B, 16C, 16D) concerned using the corresponding group of injectors (15A, 15B, 15C, 15D) according to a determined fuel quantity setpoint (Q inj,A, Q inj,B, Q inj,C, Q inj,D,), c) estimating an estimated value (P 2,e) of a second pressure in the said distribution rail at the end of step b), as a function of the said measured value (P 1,m) of the first pressure, d) acquiring a measured value (P 2,m) of the said second pressure, e) comparing the said estimated value (P 2,e) with the said measured value (P 2,m) of the second pressure, and deducing an error factor (α inj,A, α inj,B, α inj,C, α inj,D) associated with the said cylinder (16A, 16B, 16C, 16D) concerned as a function of the difference between the estimated value (P 2,e) and the measured value (P 2,m) of the said second pressure and of the said fuel quantity setpoints (Q inj,A, Q inj,B, Q inj,C, Q inj,D) and - a second operation of comparing the error factors (α inj,A, α inj,B, α inj,C, α inj,D) associated with the said cylinders (16A, 16B, 16C, 16D) and detecting a potential malfunctioning of at least one of the said groups of injectors (15A, 15B, 15C, 15D).