ECU Guides

PSA BSI and ECU Pairing Explained: Why a Peugeot or Citroen Won’t Start After an ECU or BSI Change

2 October 2026 3 min read

On Peugeot, Citroen and DS models the BSI (Boîtier de Servitude Intelligent, the body computer) is the heart of the electrical system: it reads the key transponder, controls the fuel pump, the lighting, the central locking and the diagnostic gateway. The engine ECU is paired to the BSI. Understanding PSA BSI ECU pairing saves hours when a car refuses to start after either unit has been replaced.

How the pairing works

At the first key-on after production, the engine ECU stores the immobilizer code of the BSI (“ECU coded”). From then on every start requires a valid exchange. A used ECU from another car carries a different code: the engine cranks but the injectors stay closed. Diagbox shows the ECU as coded and, on the immobilizer screen, a mismatch fault (P1517 on many EDC16/SID units).

Situation 1: the engine ECU was replaced

  • New ECU from PSA — delivered uncoded; Diagbox codes it to the car with the key present. Injector codes (C2I / IMA) must be entered afterwards on common-rail diesels.
  • Used ECU — arrives coded to the donor. It needs a virgin file to return to the uncoded state, then the Diagbox coding. Alternatively an immo off file removes the check entirely.

Situation 2: the BSI was replaced

A used BSI is coded to its own car and its own keys. It must be reset (“virginised”) with a BSI tool or EEPROM edit, then configured (VIN, options) and paired with the keys and the engine ECU by Diagbox. Fitting a used BSI without this step produces a car with no start and often no dashboard communication at all.

Situation 3: the BSI is dead

Water in the passenger footwell, a jump-start gone wrong or a failed BSI power stage are common on 2000–2012 PSA cars. Symptoms: intermittent electrics, no communication, no fuel pump. Options are a repaired or replacement BSI, or — where the BSI is obsolete — an immo off engine ECU file plus a separate fuel pump feed. Remember: immo off alone does not run the fuel pump; the BSI relay normally does that.

The fuel pump trap

Many “the immo off file doesn’t work” reports on PSA cars turn out to be a fuel pump not powered by the BSI. Check for pump priming at key-on. On the 1.6 HDi (DV6) the low-pressure pump is BSI-controlled; no prime means no start even with a perfectly coded ECU.

Which ECUs are involved

Depending on engine and year: Bosch EDC15C2, EDC16C34, EDC16C3/EDC16C39, EDC17C10, MED17.4 (petrol THP), Siemens SID803/SID803A, SID807EVO, Delphi DCM3.5 and the current Bosch MD1CS003. All are listed by software number under Peugeot and Citroen.

Checklist before ordering a file

  1. Does the BSI communicate and does the fuel pump prime? If not, fix the BSI first.
  2. Is the ECU new (uncoded), used (coded elsewhere) or original (coded to this car)?
  3. Used ECU + working BSI → virgin file + Diagbox coding. Dead/obsolete BSI → immo off.
  4. Photograph the ECU label and send the read with the file request form if the software number is not listed.
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