BMW E90 3-Series EWS Immobilizer Faults Common Symptoms and Repair

The EWS (Elektronische Wegfahrsperre) immobilizer on the BMW E90 3-Series is the anti-theft system that links the key transponder, the CAS module, and the DME — and when it fails, the car simply refuses to start. Understanding what triggers these faults can save hours of misdiagnosis.

On the E90, unlike older BMW generations that used a standalone EWS control unit, the immobilizer function is integrated into the CAS (Car Access System) module. This changes the diagnostic approach significantly compared to E46 or E36 owners who are used to looking for a separate EWS box under the dashboard.

What Does the E90 EWS / CAS Immobilizer Actually Do?

The CAS module on the E90 reads the transponder chip in the key, verifies the rolling code, and only then sends a release signal to the DME allowing the engine to start. If any part of this chain is broken — a damaged key, a failing CAS, poor antenna ring contact, or a DME that has lost its coding — the car cranks but will not fire, or in some cases will not even crank at all.

The system also controls door locking, the start button, and comfort access on equipped models. So a CAS fault often shows up as more than just a no-start; you may see central locking issues or the start button simply not responding. For comparison, see BMW E46 3-Series EWS Immobilizer Faults: Symptoms and Repair.

Common Symptoms of E90 Immobilizer Failure

Engine cranks normally but will not start — no fuel, no ignition
Dashboard shows “Drive moderately” or “No start” warning
Key symbol illuminated on the instrument cluster
Start/stop button flashes or does not respond to key proximity
Intermittent starts — car starts fine some days, refuses on others
Central locking stops working alongside the no-start condition
Car starts but immediately stalls after a second or two

That last symptom — starts and immediately cuts out — is a classic CAS/DME sync fault. The DME receives the initial start command but the immobilizer code handshake fails mid-cycle.

Common Fault Codes for E90 EWS / CAS Issues

Fault Code	Description
A804C	CAS: No communication with DME — immobilizer handshake failed
A804D	CAS: Key not recognized or transponder read error
A8050	CAS: Rolling code synchronization fault
A8060	CAS: Internal EEPROM memory fault
280B	DME: EWS / immobilizer tamper detection triggered
2A9A	DME: CAS communication fault — no release signal received

These codes are read via BMW-specific diagnostic tools — ISTA, INPA, or equivalent. Generic OBD readers will often miss CAS-level faults entirely, which leads a lot of owners to the wrong conclusions about fuel or ignition faults.

Root Causes — Why Does the E90 CAS Fail?

The most common cause is moisture ingress. On early E90s (2005–2008), the CAS module sits in the footwell area and is vulnerable to water leaks from the windscreen seal or sunroof drainage. Once moisture gets in, the internal solder joints on the EEPROM and microcontroller corrode. The failure is often intermittent at first, then becomes permanent.

Battery voltage spikes are another culprit — if someone jump-starts the car incorrectly or installs a battery without clearing the system, the CAS can lose its stored immobilizer coding. Dead keys are also frequently misdiagnosed as CAS failures; always start by testing the battery in the key fob and checking the antenna ring around the ignition before condemning the module itself. A related diagnostic guide is BMW E39 5-Series EWS Immobilizer Faults: Symptoms and Repair.

OEM Part Numbers for BMW E90 CAS Module

The correct CAS part number depends on the engine variant and production year. Common OEM references include:

61.35-9 127 443 — CAS3 unit, E90 318i / 320i, 2005–2007
61.35-9 127 444 — CAS3 unit, E90 325i / 330i, 2005–2007
61.35-9 290 794 — CAS3+ unit, E90 LCI, 2008–2011
61.35-9 290 795 — CAS3+ with comfort access, E90, 2008–2011
61.35-6 943 504 — CAS3 E90 320d / 318d diesel variant

Always match by the last 7 digits of your existing module’s label. CAS units are VIN-coded from the factory, so a replacement must be either professionally recoded or sourced pre-coded to your chassis — something GiantParts specializes in for European and worldwide customers.

Affected Vehicles

The CAS3 and CAS3+ immobilizer system is shared across the entire E9x generation:

BMW E90 3-Series Sedan, 2005–2011 (all engines)
BMW E91 3-Series Touring, 2005–2011
BMW E92 3-Series Coupe, 2006–2012
BMW E93 3-Series Convertible, 2007–2012
BMW E87 / E81 1-Series (shares CAS3 architecture, different part numbers)
BMW E84 X1 (2009–2012, CAS3+ variant)

The fault patterns are essentially the same across all these variants, though the exact module part number changes by body style and equipment level.

Repair vs Replacement — What Makes Sense?

If the CAS has internal EEPROM corruption from moisture, specialist electronics repair shops can sometimes rework the board and re-flash the data — but success rates vary. Physical corrosion damage is rarely worth repairing because the board degradation continues. A tested used OEM unit, properly matched and coded to the VIN, is the more reliable and often more cost-effective route. Buying a used CAS without verifying it can be coded to your chassis is a common and expensive mistake — always confirm before purchasing.

Frequently Asked Questions

Why does my BMW E90 crank but not start and show a key warning light?

The key warning light on the E90 instrument cluster indicates the CAS module has failed to read or verify the transponder in your key. This is most commonly caused by a dead key battery, a faulty antenna ring, or a failing CAS module. Start by replacing the key battery and testing with a spare key before further diagnosis.

Can I replace the BMW E90 CAS module myself?

Physically removing and fitting the module is straightforward — it is held by two screws in the lower dashboard. However, the replacement CAS must be coded to your vehicle’s VIN using BMW ISTA or a compatible tool; without this step the car will not start with the new unit. Most owners have this done at a workshop or source a pre-coded unit matched to their chassis. For additional context, read BMW E38 7-Series EWS Immobilizer Faults Symptoms and Repair.

Will a second-hand CAS from another E90 work in my car?

Not directly out of the box. The CAS stores vehicle-specific data including the VIN and the DME synchronization codes. A used unit must be recoded or adapted to your specific vehicle. Suppliers like GiantParts can help match and prepare units before shipping so the coding process at the workshop is simplified.

What is the difference between CAS3 and CAS3+ on the E90?

The CAS3 was used on pre-LCI E90 models (2005–2008) and handles basic immobilizer and central locking. The CAS3+ introduced on the LCI (2008 onwards) added additional functions including improved comfort access processing and enhanced communication protocols. The two versions are not directly interchangeable without software adaptation.

How do I know if my E90 no-start is the CAS or the DME?

Connect a BMW-compatible diagnostic tool and check whether fault codes are stored under the CAS module or the DME. A fault in the CAS (transponder read errors, internal memory faults) points to the CAS. Fault code 2A9A or 280B stored in the DME with no CAS codes often suggests a lost synchronization between the two modules, which may require resyncing rather than full replacement. In some cases both modules need attention.

Find tested OEM electronic parts and modules at:
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