Almost all peugeots and citroens since the year 2000 have a "BSI" Body Systems Interface unit, this unit is the electronic brain for the vehicle and controls most of the essential functions such as remote control codes, immobiliser coding, radio paring, dash paring, engine ecu paring, timer relays, vehicle mileage, service information, vehicle type, engine type, central locking and various internal relays and transistors. This information has to be programmed into the BSI and paired to all the other ECUs in the vehicle, this can be over 20 pages of information and requires the access PIN code to allow configurations and options. Once the unit has been programmed it cannot be fitted to another vehicle, this means that a second hand unit is useless.

The most common reasons for Peugeot BSI and Citroen BSI failures are water ingress, flat batteries, jump starting or poor wiring, most modern vehicles have BSI modules fitted to them, the first step is proving that the BSI is actually faulty and not a wiring fault or component failure.