BMW E90 3-Series CAS Module Failure: Symptoms and Repair Guide

The CAS (Car Access System) module in the BMW E90 3-Series is a small but absolutely central control unit — it manages key recognition, immobilizer communication, engine start authorisation and interior access. When it fails, the car simply refuses to start, and diagnostics without the right knowledge can quickly turn into a wild goose chase.

This guide is aimed at workshop technicians and BMW owners dealing with CAS-related no-start faults, key recognition errors or erratic central locking on E90 models built between 2005 and 2012.

What Exactly Does the CAS Module Do?

The CAS module is essentially the gateway between the driver and the powertrain. It reads the key transponder signal, communicates with the EWS-style immobilizer logic, sends a release signal to the DME (Digital Motor Electronics), and manages the start/stop button on keyless entry variants. On E90s it also handles door unlocking sequences and communicates over the K-CAN bus with several other modules.

In simple terms: no CAS signal, no start. It’s the bouncer at the door of every drive cycle.

Common Symptoms of CAS Module Failure

Engine cranks but will not fire — immobilizer remains active even with the correct key
Key not recognised warning in the instrument cluster
Start/stop button pressing with no response whatsoever
Central locking operating intermittently or not at all
Random loss of all interior electrics for a few seconds after inserting the key
Battery drain overnight despite no obvious load left on
CAS fault codes stored even after key replacement

Worth noting: a dead key battery will mimic CAS failure almost perfectly. Always swap in a fresh CR2032 first. If the problem persists after a fresh battery, the CAS itself is worth serious attention. For comparison, see BMW E90 3-Series CAS Module Failure: Symptoms and Repair Guide.

Common Error Codes — E90 CAS Faults

Fault Code	Description
A804F	CAS: No communication with DME/DDE — CAS not releasing engine start
A802F	CAS internal fault — module self-test failure
A806F	CAS: Transponder communication error — key not authenticated
A807F	CAS: Bus communication fault on K-CAN
A80AF	CAS: Voltage supply fault — low or intermittent power to unit
480101	CAS immobilizer active — engine start blocked

These codes appear in BMW-specific scan tools such as ISTA/D, INPA or Carly. Generic OBD2 readers often miss them entirely because CAS communicates on a proprietary bus, not the standard OBD gateway.

Why Does the CAS Module Fail on E90s?

The E90 CAS is particularly vulnerable to two things: moisture ingress and voltage spikes. The module sits behind the dashboard on the driver’s side — not the most exposed location, but a leaking pollen filter housing or a previous screen seal failure can push enough damp air into the cabin to corrode the CAS connector pins over years. Corrosion on the 26-pin CAS connector is one of the most common root causes we see in the workshop.

Voltage spikes from improper jump-starting are the second killer. E90s do not like being jump-started from high-capacity truck batteries or fast chargers without proper current limiting. The CAS is one of the first casualties when an overvoltage event hits.

Finally, on higher-mileage cars (above 150,000 km), simple age-related EEPROM failure inside the unit is not unusual. The internal memory chip holding the vehicle-specific immobilizer data becomes unreliable over time.

Affected Vehicles

BMW E90 3-Series Saloon — 2005–2011 (318i, 320i, 323i, 325i, 330i, 320d, 325d, 330d)
BMW E91 3-Series Touring — 2005–2012
BMW E92 3-Series Coupe — 2006–2012
BMW E93 3-Series Convertible — 2007–2012
BMW E87 1-Series — shares the same CAS platform and is susceptible to identical faults
BMW E82/E88 1-Series Coupe/Convertible — same CAS generation

The CAS3 and CAS3+ variants are used across this generation. It is important to identify which version is fitted before sourcing a replacement, as they are not interchangeable without recoding. A related diagnostic guide is BMW E90 3-Series DME Engine Control Module Faults Diagnosis.

Common OEM Part Numbers — BMW E90 CAS Module

Oryginalny numer części	Application
61.35-9-180-414	CAS3 unit — E90/E91 petrol, pre-facelift
61.35-9-291-591	CAS3 unit — E90/E91/E92 facelift, common fit
61.35-6-942-547	CAS3 — E87 1-Series, often cross-referenced to E90 early
61.35-9-335-405	CAS3+ unit — E92/E93 with keyless entry option
61.35-9-180-421	CAS3 — E90 diesel variants, 320d/325d

Always cross-reference the part number from the original unit label before ordering. CAS modules are VIN-locked from the factory, so a tested used OEM unit will need to be coded to the vehicle using ISTA/P or equivalent BMW programming software.

Repair vs Replacement — What Makes Sense?

If the fault is connector corrosion, cleaning and re-pinning the 26-pin connector is worth attempting first — it costs very little and occasionally resolves the issue completely. Use electrical contact cleaner and apply dielectric grease after reassembly.

If the module itself has failed internally, repair is theoretically possible through a specialist who can re-flash or rework the EEPROM chip, but the success rate varies and turnaround time can stretch to a week. For most workshops, a tested used OEM CAS unit with known-good coding history is the faster and more reliable path. The key requirement is that the replacement module is matched to the car’s ISN (Individual Serial Number) and correctly synchronised with the DME and instrument cluster — this is not a plug-and-play swap.

FAQ

Can I replace the BMW E90 CAS module myself?

Physically removing and fitting the unit is straightforward — it is held by two screws and a connector behind the dashboard. However, the module must be programmed to the vehicle’s ISN using BMW ISTA/P software or an equivalent tool. Without correct coding, the replacement CAS will not release the immobilizer and the car will still not start. This step requires specialist equipment. For additional context, read BMW E90 3-Series ABS Control Module Failure: Symptoms and Repair.

Will a used CAS module work in my E90?

A tested used OEM CAS module can absolutely work, but it must be sourced from a compatible variant (CAS3 or CAS3+ as appropriate) and then coded to your specific vehicle. A reputable supplier will provide the unit’s original part number and ideally confirm the CAS version. At GiantParts.eu, all tested OEM modules are checked before dispatch and listed with part numbers to simplify matching.

What scan tool do I need to diagnose E90 CAS faults?

BMW-specific diagnostic software is required — ISTA/D (Rheingold), INPA or a professional-grade tool such as Autel MaxiSYS with BMW CAS coverage. Generic OBD2 readers do not access the CAS module because it sits on the proprietary K-CAN bus, not the standard OBD gateway. A cheap ELM327 reader will show nothing useful here.

How do I know if it is the CAS module or just the key fob?

Try the spare key first — if the second key starts the car, the primary key fob or its transponder has failed, not the CAS. If neither key works and fresh batteries have been fitted to both fobs, fault codes A806F or A804F point clearly toward the CAS module itself rather than the keys.

Is the E90 CAS fault covered under any BMW recall?

There is no blanket BMW recall for E90 CAS module failures. Some early production cars received dealer attention for key recognition software bugs under technical service bulletins, but by this point most E90s are well outside any warranty coverage. The repair is the owner’s responsibility, and a tested used OEM replacement is typically the most cost-effective solution available.

Find tested OEM electronic parts and modules at:
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BMW E90 CAS Module Failure: Symptoms and Repair