A motor vehicle diagnostics apparatus having a diagnostics unit connectable in-line by means of first and second connector ports (112, 114) in a motor vehicle control system between first and second components (171, 172) via first and second cables, each have a respective connector (122C, 124C), the apparatus being operable to detect by means of the diagnostics unit one or more electrical signals generated by the first component for application to the second component and to provide an output in response to the detected signals, wherein the output provides one or more of: an indication whether the detected signals correspond to expected signals by reference to data in respect of expected signals, a visual indication of a correspondence between a form of a signal detected by the diagnostics unit and an expected form of the signal; and/ or a visual or audible indication whether a value of at least one characteristic of a detected signal is within a prescribed range of values of that characteristic, characterized in that the apparatus is operable to perform a self test of the first and second cables and the first and second connectors associated therewith in which, in use: the first connector is connected to the second connector and a prescribed signal is transmitted from the first connector port to the second connector port via said first and second cables and connectors associated therewith; the diagnostics unit is operable to detect one or more corresponding signals received at the second connector port after application of the prescribed signal applied at a signal line at the first connector port, detection of the corresponding signal indicating that the applied signal has been successfully transmitted by the first and second cables. the apparatus being operable to detect by means of the diagnostics unit one or more electrical signals generated by the first component for application to the second component and to provide an output in response to the detected signals, wherein the output provides one or more of: an indication whether the detected signals correspond to expected signals by reference to data in respect of expected signals, a visual indication of a correspondence between a form of a signal detected by the diagnostics unit and an expected form of the signal; and/ or a visual or audible indication whether a value of at least one characteristic of a detected signal is within a prescribed range of values of that characteristic, characterized in that the apparatus is operable to perform a self test of the first and second cables and the first and second connectors associated therewith in which, in use: the first connector is connected to the second connector and a prescribed signal is transmitted from the first connector port to the second connector port via said first and second cables and connectors associated therewith; the diagnostics unit is operable to detect one or more corresponding signals received at the second connector port after application of the prescribed signal applied at a signal line at the first connector port, detection of the corresponding signal indicating that the applied signal has been successfully transmitted by the first and second cables.