A method for monitoring of an After Treatment Injector located in an exhaust gas system of an internal combustion engine, said exhaust gas system comprising: - an exhaust gas purification unit, - an After Treatment Substance Injection System (ATSIS) including a) an After Treatment Injector for the injection of a substance into the exhaust gas system, b) a conduit for supplying of the substance from a container (5, 12) to the ATI, c) a pressure sensor located in or near the ATI in such a way it will sense the pressure of the substance in the ATI, d) an Electronic Control Unit for control of the ATI, - an On Board Diagnosing system which is connected to the ECU and the pressure sensor, either directly or via the ECU, said OBD being provided with comparative data for deciding the functionality of the After Treatment Injector in order to detect abnormal functionality of the ATI, characterized in that - said ECU is programmed to perform a predetermined sequence of opening and closing commands for the ATI with a specific frequency having a specific corresponding pressure frequency signal which is indicating normal injection function, wherein a corresponding signal variation is expected from the pressure sensor, and - said OBD programmed to perform frequency analysis of the measured pressure values detected by said pressure sensor in order to prepare a pressure curve, and comprising - comparing said pressure curve with one or several reference curves stored in the OBD in order to decide the functionality of the ATI, wherein the pressure sensor indicates both amplitude and timing of pressure oscillation which is compared with the expected shape of the pressure characteristics corresponding to the expected fuel delivery from the ATI according to a defined duty or test cycle. - an exhaust gas purification unit, - an After Treatment Substance Injection System (ATSIS) including a) an After Treatment Injector for the injection of a substance into the exhaust gas system, b) a conduit for supplying of the substance from a container (5, 12) to the ATI, c) a pressure sensor located in or near the ATI in such a way it will sense the pressure of the substance in the ATI, d) an Electronic Control Unit for control of the ATI, a) an After Treatment Injector for the injection of a substance into the exhaust gas system, b) a conduit for supplying of the substance from a container (5, 12) to the ATI, c) a pressure sensor located in or near the ATI in such a way it will sense the pressure of the substance in the ATI, d) an Electronic Control Unit for control of the ATI, - an On Board Diagnosing system which is connected to the ECU and the pressure sensor, either directly or via the ECU, said OBD being provided with comparative data for deciding the functionality of the After Treatment Injector in order to detect abnormal functionality of the ATI, characterized in that - said ECU is programmed to perform a predetermined sequence of opening and closing commands for the ATI with a specific frequency having a specific corresponding pressure frequency signal which is indicating normal injection function, wherein a corresponding signal variation is expected from the pressure sensor, and - said OBD programmed to perform frequency analysis of the measured pressure values detected by said pressure sensor in order to prepare a pressure curve, and comprising - comparing said pressure curve with one or several reference curves stored in the OBD in order to decide the functionality of the ATI, wherein the pressure sensor indicates both amplitude and timing of pressure oscillation which is compared with the expected shape of the pressure characteristics corresponding to the expected fuel delivery from the ATI according to a defined duty or test cycle. - said ECU is programmed to perform a predetermined sequence of opening and closing commands for the ATI with a specific frequency having a specific corresponding pressure frequency signal which is indicating normal injection function, wherein a corresponding signal variation is expected from the pressure sensor, and - said OBD programmed to perform frequency analysis of the measured pressure values detected by said pressure sensor in order to prepare a pressure curve, and comprising - comparing said pressure curve with one or several reference curves stored in the OBD in order to decide the functionality of the ATI, wherein the pressure sensor indicates both amplitude and timing of pressure oscillation which is compared with the expected shape of the pressure characteristics corresponding to the expected fuel delivery from the ATI according to a defined duty or test cycle.