Abstract: A diagnostic/control device for fuel injected internal combustion engines comprising a control system (14) connectable via leads (16) to the fuel injectors (10) after disconnection therefrom of an engine management computer (11), the latter being connected directly (at 18) to the control system (14). An oxygen sensor (13) is connected (at 17) to the control system (14) whereby the latter is connected between the engine management computer (11) and the fuel injectors (10) and can be operated in closed circuit or open circuit mode to control the injectors directly when required independently of the engine management computer (11).