The application of increasingly complex systems on board causes problems on their compatibility inside the electromagnetic car environment. A study was carried out to study the electromagnetic behaviour of some car models. Results show that the car's electrical impedance may be simulated by an equivalent network, easily simulated in the laboratory. To define the operating limits of an electronic engine control system (CEM), we connected this system to the equivalent car network and studied the effects of induced electromagnetic noise which simulate present conditions. Results have generated a proposal of E.M.C. Standards for the automotive field.