A method is provided for monitoring of the functionality of an After Treatment Injector (ATI) for injection of a fluid in an engine exhaust gas system upstream of an exhaust gas treatment unit. The monitoring is performed by using a pressure sensor which detects the pressure of the fluid delivered to the ATI for a specific condition or injection cycle. The pressure sensor is located downstream of a flow regulator and upstream of the ATI in a conduit for supplying of a substance from a container to the ATI. An ECU is programmed to perform a predetermined sequence of opening and closing commands for the flow regulator and the ATI in order to monitor the functionality of the flow regulator, the ATI and the pressure sensor. The can be used in connection with an On Board Diagnosis (OBD) system.