BMW CAS / DME / DDE Start Authorisation — Immobiliser & Start-Permission Diagnosis

The role of the start-authorisation system

BMW start authorisation is the immobiliser and start-permission chain that lets the DME/DDE enable injection. The engine cannot start unless every link in the chain agrees: the key is recognised, the authorisation module and the DME/DDE are aligned, and the start-permission message reaches the engine controller over CAN. When the chain breaks, the engine cranks but will not fire — even when fuel, ignition, and mechanical systems are completely healthy.

This page is for legitimate repair diagnosis only. It does not provide instructions for bypassing immobilisers, defeating security, cloning keys, or any unauthorised key or module access.

Trademark notice: MechanIQ is independent software and is not affiliated with, endorsed by, or sponsored by BMW AG. "BMW", "CAS", "FEM", "BDC", "DME", "DDE", "EWS", "ISTA" and related terms are trademarks of their respective owners.

CAS / FEM / BDC by generation

The authorisation module changed across BMW generations:

• EWS / CAS (CAS2, CAS3) — E-series (E90, E70, and earlier). EWS is the earlier immobiliser; CAS consolidated immobiliser, start authorisation, and terminal-15 wake-up.
• CAS-4 / FEM / BDC — F-series and later (F30, F10, and later). The body domain controller (FEM, later BDC) handles authorisation and start permission in concert with CAS-4.

Confirm the exact generation and module from the vehicle identification before testing — the evidence path differs.

DME / DDE start permission

The DME/DDE will not enable injection until it receives a valid start-permission message from the authorisation module. This is not a fuel-pressure decision; it is a security gate. If the permission message does not arrive or is not trusted, requested rail pressure stays at zero and injection is inhibited — the engine cranks but does not fire.

Key recognition

The key must be recognised by the authorisation module. A key that is not recognised (damaged transponder, un-programmed key, a key from another vehicle) blocks start permission. Confirm with a known-good, already-paired key where available. This is legitimate diagnosis — not key cloning or unauthorised access.

Communication between relevant modules

Start permission travels over CAN between the authorisation module (CAS/FEM/BDC) and the DME/DDE. A CAN fault between these modules blocks the permission message and produces a crank/no-start that is not a fuel or mechanical fault. Confirm the CAN path between authorisation and engine controller before condemning either module. See the CAN communication guide.

Synchronisation / alignment concept

The authorisation module and the DME/DDE share an alignment (sometimes called synchronisation) that lets them trust each other's messages. If the alignment is lost — after a module replacement, a failed update, or a corrupted memory — the DME/DDE will not trust the start-permission message and the engine will not start. Alignment is restored with legitimate, BMW-authorised service procedures and tooling (ISTA), not by bypass. Vehicle-specific verified procedure required for any alignment operation.

Why crank / no-start occurs with healthy fuel and mechanical

A crank/no-start caused by authorisation has a distinctive signature: the engine cranks at normal speed, the fuel system is capable (low-pressure supply present, HP pump functional), the crank/cam signals are intact, compression is good — yet requested rail pressure stays at zero and injection is inhibited. The engine is not failing to run; it is not being permitted to run.

How scan-tool evidence is used

ISTA exposes the authorisation and start-permission status. Read:

• Key recognition status — is the key recognised?
• Start-permission status — is permission granted to the DME/DDE?
• Authorisation / DME-DDE alignment status — is the alignment intact?
• CAN communication status between the modules

These statuses direct the diagnosis before any fuel or mechanical testing.

Differentiating authorisation from rail-pressure and crank-signal faults

The three crank/no-start causes that all inhibit injection must be separated:

• Authorisation fault — requested rail pressure stays at zero, but key/permission status fails. Fuel system is capable.
• Crank/cam signal loss — requested rail pressure stays at zero, but the authorisation status is OK. The engine-speed signal is the fault.
• Rail-pressure fault — authorisation OK, crank/cam OK, requested rail pressure is commanded but actual cannot follow. The fuel system is the fault.

The requested-rail-pressure behaviour plus the authorisation status plus the crank/cam scope separate the three. See the cranks-but-won't-start guide for the full sequence.

Legitimate-repair boundary

This page covers diagnosis of authorisation faults for legitimate repair: confirming key recognition, CAN communication, alignment status, and module integrity. It does not cover, and MechanIQ will not assist with, immobiliser bypass, security defeat, key cloning, or unauthorised module access. Any alignment or programming operation requires BMW-authorised tooling and procedure.

Verification

After legitimate repair, reproduce the start condition (cold, hot, short hot-soak restart), confirm start permission is granted and injection is enabled, and clear all modules. A no-start that does not reproduce under the original condition is not verified fixed.

Diagnostic CTA

Diagnosing a BMW start-authorisation fault on a real vehicle? Import your ISTA start-permission and fault memory into MechanIQ and let the evidence engine separate authorisation from fuel and signal faults.

Trademark disclaimer

MechanIQ is independent software. BMW, CAS, FEM, BDC, DME, DDE, EWS and ISTA are trademarks of BMW AG. MechanIQ is not affiliated with or endorsed by BMW AG. Alignment and programming require authorised tooling and procedure; vehicle-specific verified procedure required.