Almost all Peugeot, Citroen and Renault vehicles since the year 2000 have a BSI module - Body Systems Interface unit or UCH interconnection unit, these are the electronic brains for the vehicles and control 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 , indicators, hazards, wipers, washers, electric windows and also 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 changes. Once the unit has been programmed it cannot be fitted to another vehicle, this means that a second hand unit is useless.
A brand new UCH is supplied with a blank memory. This must be coded (using the Renault Clip computer) to match the serial number of the ECU and key/card before it will work with the car. The memory can only be written once, so a used UCH cannot be taken from a donor vehicle to be used in another unless the ECU and keyfobs/cards are also transferred.
The most common reasons for Peugeot BSI, Citroen BSI and Renault UCH interconnection unit 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 or Renault UCH is actually faulty and not a wiring fault or a component failure.
Volkswagen, Audi, Seat and Skoda also have various comfort modules and control units which can be replaced and configured too!