BMW CAS Module Repair: Car Access System Faults, Symptoms & Solutions

4 days ago
3 min read

What Does the BMW CAS Module Do?

The CAS module — Car Access System — is the central gateway for vehicle security and starting on modern BMWs. It sits behind the dashboard near the steering column and handles critical functions:

Immobiliser control — authenticates your key before allowing engine start
Key recognition — identifies which key is being used (up to 10 per vehicle)
Starter relay activation — directly controls the starter motor relay
EWS integration — from CAS2 onwards, immobiliser functions are built in
Steering lock control — communicates with Electronic Steering Lock on keyless models
Central locking and comfort access

CAS Module Variants

CAS1 — E65/E66 7 Series (2001-2005). Original unit, now rare but repairable.
CAS2 — E60/E61 5 Series, E63/E64 6 Series (2003-2007). First integrated EWS.
CAS3 — E90/E91/E92/E93, E87/E81, E60 LCI, E70 X5 (2005-2010). Most common variant we repair.
CAS3+ — F30, F10, F20 series (2009-2014). Updated encryption.

Symptoms of a Failing CAS Module

Car won't start with a valid key — dashboard lights up but no crank. Starter relay inside CAS has failed.
Key fob not recognised — "Key Invalid" or "No Key Detected" message.
No crank, no start — often misdiagnosed as starter motor or battery fault.
Intermittent starting — works some days, refuses others. Classic dry joint or corrupted EEPROM.
All dashboard warnings — CC-ID 169 (drive moderately, multiple faults) from CAN bus communication errors.
Steering lock fault — "Steering lock defective" on E90 and E60. Faulty CAS often the root cause.
Fault codes: 2F42 (CAS internal fault), 2F43 (starter relay), A0B4 (key auth failure), A0A1 (EWS tamper detected).

What Causes CAS Module Failure

Internal Relay Failure

The starter relay is soldered directly to the CAS board. After thousands of start cycles, solder joints crack or relay contacts wear. Number one cause of "no crank" on CAS3 modules.

EEPROM Corruption

Key data, ISN (Individual Serial Number), and vehicle config are stored in onboard EEPROM. Power surges from a failing battery or botched jump-start corrupt this data. In many cases, EEPROM can be read, corrected, and rewritten.

Water Damage from A-Pillar Leaks

Well-known BMW problem on E90 and E87 models. Blocked sunroof/windscreen drain channels route water down the A-pillar onto the CAS module. We've opened units with visible corrosion inside.

Component Ageing

Electrolytic capacitors degrade in the warm environment behind the dashboard, causing voltage regulation issues and intermittent CAN bus faults.

Repair vs Replacement: The Numbers

BMW main dealer replacement typically involves: New CAS module £500-£800. Coding and programming £200-£400. Key programming £150-£250 per key. DME ISN alignment. All in: £1,500 to £2,500.

A specialist CAS module repair costs £150 to £250. Your original module is returned with all key data, ISN, and vehicle coding intact. In most cases, the car starts straight away when refitted.

Our Postal CAS Module Repair Service

1. Get in touch — WhatsApp +44 7404 487674 or order via ecu-repair.co.uk. Describe your symptoms and we'll confirm if CAS repair will sort it.

2. Remove and send your CAS module — two bolts and a wiring connector. We can talk you through removal.

3. Bench testing — tested on our dedicated CAS bench rig. Exact fault confirmed before work begins.

4. Repair and reprogramming — faulty components replaced, EEPROM data verified and corrected.

5. Return within 2-3 working days. Plug it back in and you should be running.

All repairs come with a lifetime warranty.

Important: Key Re-Syncing After CAS Repair

In some cases, particularly where EEPROM data was repaired, your keys may need re-syncing after refitting. This is a quick procedure with ISTA, BimmerLink, or similar tools. A local BMW specialist or auto locksmith can do this in minutes. We'll let you know if this applies before returning the module.

Frequently Asked Questions

Can I drive my BMW with a faulty CAS module?

If the car starts at all, you'll have reduced functionality and intermittent faults. You could easily end up stranded. Best to sort it sooner rather than later.

How do I know if it's the CAS and not the key or battery?

Try a second key. If both produce the same symptoms, it's not the key. Battery at 12.4V+ rules out power issues. Diagnostic codes 2F42/2F43/A0B4 point firmly at the CAS. Send us your symptoms and fault codes via WhatsApp.