Signalling system for a motor vehicle, comprising: - a first light module which is able to perform part of a first regulatory signalling function; - a second light module which is able to perform part of a second regulatory signalling function which is distinct from the first function, and - a third light module which is able to selectively perform at least one other part of the first regulatory signalling function and at least one other part of the second regulatory signalling function; the signalling system being characterized: - in that it comprises bidirectional communication means (113, 123, 133), notably wired ones, connecting the first light module to the third light module and the second light module to the third light module, - in that each of the first, second and third light modules (110, 120, 130) comprises diagnostic means (112, 122, 132) which are able to diagnose a failure of at least one of the components of the first, second and third light modules, respectively, notably a failure of a light source of the first, second and third light modules, respectively, - in that the third light module comprises control means, arranged to order the deactivation of the emission of light from the first, second and third light modules (110, 120, 130), - in that the diagnostic means of the first light module are able to transmit an item of information relating to the status of the first light module to the control means via these bidirectional communication means, - in that the diagnostic means of the second light module are able to transmit an item of information relating to the status of the second light module to the control means via these bidirectional communication means, - in that the control means are arranged to order the deactivation of the emission of light from the third light module only for performing the first regulatory signalling function when the third light module receives an item of information from the first light module relating to a failure of the first light module, and to order the deactivation of the emission of light from the third light module only for performing the second regulatory signalling function when the third light module receives an item of information from the second light module relating to a failure of the second light module, and - in that the signalling system is configured so that the emission of light from the first and second light modules (110, 120) is deactivated in the event of a failure of the third light module. - a first light module which is able to perform part of a first regulatory signalling function; - a second light module which is able to perform part of a second regulatory signalling function which is distinct from the first function, and - a third light module which is able to selectively perform at least one other part of the first regulatory signalling function and at least one other part of the second regulatory signalling function; - in that it comprises bidirectional communication means (113, 123, 133), notably wired ones, connecting the first light module to the third light module and the second light module to the third light module, - in that each of the first, second and third light modules (110, 120, 130) comprises diagnostic means (112, 122, 132) which are able to diagnose a failure of at least one of the components of the first, second and third light modules, respectively, notably a failure of a light source of the first, second and third light modules, respectively, - in that the third light module comprises control means, arranged to order the deactivation of the emission of light from the first, second and third light modules (110, 120, 130), - in that the diagnostic means of the first light module are able to transmit an item of information relating to the status of the first light module to the control means via these bidirectional communication means, - in that the diagnostic means of the second light module are able to transmit an item of information relating to the status of the second light module to the control means via these bidirectional communication means, - in that the control means are arranged to order the deactivation of the emission of light from the third light module only for performing the first regulatory signalling function when the third light module receives an item of information from the first light module relating to a failure of the first light module, and to order the deactivation of the emission of light from the third light module only for performing the second regulatory signalling function when the third light module receives an item of information from the second light module relating to a failure of the second light module, and - in that the signalling system is configured so that the emission of light from the first and second light modules (110, 120) is deactivated in the event of a failure of the third light module.