Mechanic Wiki: Fix BMW CAS4 All Keys Lost with LAUNCH X431 IMMO Plus via OBD

Step-by-step guide to program new keys for BMW CAS4 immobilizer using LAUNCH X431 IMMO Plus in OBD mode. Includes prerequisites, troubleshooting, and CAS4+ tips.

Quick Answer: Fixing a BMW CAS4 "All Keys Lost" with LAUNCH X431 IMMO Plus via OBD

Yes, you can program new keys for a BMW CAS4 immobilizer system using the LAUNCH X431 IMMO Plus tool in OBD mode without removing the CAS module. The process involves reading the CAS4 MCU (usually NEC or Renesas), generating dealer keys, and synchronizing transponders. Success depends on tool version, vehicle power stability, and correct OBD wiring. For CAS4+ (revised) modules, OBD mode may not work – you’ll need bench mode.

Understanding the "All Keys Lost" Scenario in BMW CAS4

When you face an "All Keys Lost" situation on a BMW with a CAS4 module, it means no valid transponder is recognized by the immobilizer system. This typically happens when all original keys are missing, damaged, or their electronic chips have failed. The CAS module locks itself into a state where it refuses to accept new keys without a specialized diagnostic procedure.

The LAUNCH X431 IMMO Plus has become a go-to solution for many auto locksmiths and DIY mechanics. Why? Because it combines immobilizer programming with a user-friendly interface, supports multiple protocols, and can often do the job via the OBD-II port – no need to tear apart the dashboard or solder wires to the ECU. But as with any advanced diagnostic tool, success depends on preparation and understanding the nuances of the CAS4 system.

Prerequisites: What You Need Before Starting

Before you plug anything in, gather the following tools and information. Skipping any of these can lead to a failed session – or worse, a bricked module.

Tools and materials:

LAUNCH X431 IMMO Plus (make sure the software is updated to the latest version – check for IMMO Plus database updates inside the tablet)
Stable 12V battery charger with at least 10A output – a simple trickle charger won’t cut it. Voltage drops during the write phase are the #1 cause of failures.
Blank BMW keys with the correct transponder chip. For CAS4, you’ll typically need PCF7946 (Hitag2 protocol). For CAS4+, it’s often PCF7935 or 7953. Always confirm chip type by reading the original module data before buying blanks.
Key cutter if the vehicle uses laser-cut keys (common on 3-series, 5-series from around 2005-2012).

Vehicle knowledge you must confirm:

Identify whether the car has a CAS4 or CAS4+ module. This is critical because OBD mode works only on the original CAS4. CAS4+ blocks read access through the diagnostic port. Check the part number on the CAS module (located behind the glove box or under the dash). If it ends in a revision like 61359120978, you may have CAS4+.
Ensure the ignition is mechanically sound – a stuck steering lock or faulty ignition switch can interrupt the programming sequence.

Step-by-Step Procedure: Using OBD Mode

The following steps assume your vehicle is confirmed as a CAS4 (not CAS4+). If you’re unsure, start here – if it fails, you’ll know to move to bench mode.

1. Prepare the Vehicle

Connect the battery charger directly to the battery terminals under the hood (not through the cigarette lighter). Turn the ignition ON to position 2 – you don’t need the engine running, but the dashboard should light up. If you have no working key at all, some versions of the X431 can simulate the ignition-on state through a dedicated menu. Wait about 30 seconds for all modules to wake up on the CAN bus.

2. Connect LAUNCH X431 IMMO Plus

Boot up the X431 tablet. Open the IMMO Plus application from the main menu. Navigate to BMW → CAS4 → OBD Mode. The tool will walk you through a pairing process with the VCI (Vehicle Communication Interface) dongle. Use the original LAUNCH cables – third-party OBD cables often cause communication errors.

3. Read CAS4 MCU Data

Select the "All Keys Lost" option. The tool will now attempt to read the EEPROM and Flash memory of the CAS4 module through the OBD port. This process typically takes 2 to 5 minutes. Do not touch any cables, turn off the ignition, or close the hood during this phase. If the read fails, you’ll get a communication error – move to troubleshooting below.

When successful, the screen will display the key positions and their status (e.g., "Slot 1: Empty", "Slot 2: Empty"). This confirms the tool can see the module’s memory.

4. Generate Dealer Key

From the same menu, select "Add New Key". Choose how many keys you want to program – the CAS4 can store up to 8 keys. The tool will generate a dealer key file – a binary that mimics the original manufacturer key data. Depending on your X431 version, you may be asked to write this file to a blank transponder using the tool’s built-in transponder programmer (if equipped) or an external programmer like Tango or Orange5. Follow the on-screen prompts.

5. Program Transponder to the Vehicle

Insert the now-programmed blank key into the ignition slot. Turn the ignition ON – the X431 will detect the new key and attempt to synchronize it with the CAS module. You’ll see a confirmation message like "Key Learned" on the screen. At this point, leave the ignition on for about 10 seconds, then turn it off and remove the key.

6. Test All Keys

Now test each programmed key. Insert one, turn to position 2, then start the engine. The car should crank and run normally. If remote locking functions (RKE) are missing, don’t worry – that’s a separate step. Use the X431 to go to Body Module → Remote Key Programming to learn the remote codes. Some BMW models require two working remotes to be paired at once, so if you have only one, you may need to overwrite both using the tool.

Detailed Troubleshooting Steps

Even with careful preparation, things can go sideways. Here’s how to handle the most common issues.

OBD Mode Fails – Read Error

If the tool cannot read the CAS4 MCU, the most likely culprits are:

Aftermarket alarm systems that block the K-Line or CAN communication. Try temporarily disabling any third‑party security modules.
Weak battery – even if the car starts, the voltage can drop below 12V during the read/write cycle. Use a genuine battery charger, not a jump starter pack.
Ignition not fully ON – try turning the key to position 2 before connecting the X431.
CAS4+ module – if all else fails, the module is likely CAS4+. You must remove it and use bench mode.

Key Not Learned After Programming

The tool says "Key Learned", but the car won’t start. Here’s what to check:

Wrong transponder chip – double‑check the chip type. CAS4 uses PCF7946; CAS4+ uses PCF7935 or 7953. Using the wrong chip will prevent sync.
Ignition cycle needed – some CAS4 modules require you to turn the ignition off and on twice before they accept the new key. Try removing the key, waiting 10 seconds, then reinserting and starting.
Key blade not cut correctly – laser‑cut keys need precise cutting. If the blade doesn’t turn the ignition cylinder fully, the immobilizer won’t see it.

Remote Unlock Not Working

This is a separate system. Use the X431 under Body Module → Remote Key Programming. Follow the on‑screen instructions – often you’ll need to press a button on the remote within 10 seconds of a command. If you have only one working remote, the tool may ask you to overwrite both to maintain two slots.

Additional Context & Tips

CAS4 vs. CAS4+ – Why OBD Mode May Not Work

This distinction trips up many DIYers. The original CAS4 (roughly 2005–2010) uses a Renesas RH850/F1L microcontroller that allows EEPROM reading through the OBD port. The revised CAS4+ (2010–2013) uses an NXP/NEC MCU that blocks external read access via OBD. You must remove the CAS module and use bench mode (connecting directly to the circuit board). To identify your module, look at the sticker on the CAS unit – if it ends in a letter like "A" or "B", it’s likely CAS4+. Alternatively, check the software version in the X431 after a failed read – it often tells you.

Important Safety Precautions

Never disconnect the battery charger during a write operation. Interrupting power can corrupt the flash memory and brick the CAS module. In that case, you’ll need a professional to reflash via JTAG.
Use only original LAUNCH cables. Cheap OBD cables from third parties often have different pinouts or poor shielding, leading to data corruption.
Backup the original EEPROM when the tool prompts you. The X431 usually asks before writing. Save that backup file – if something goes wrong, you can restore it later.

Frequently Asked Questions

1. Can I use this method if I have one working key left?

No – OBD mode "All Keys Lost" is designed only when no keys are recognized. If you have one key, use the "Add Key" function instead to avoid erasing existing key.

2. Do I need to remove the CAS module?

Only if the OBD mode fails or you have a CAS4+. Otherwise, OBD mode works without removal.

3. What transponder chip does BMW CAS4 use?

Most CAS4 units use PCF7946 (Hitag2 protocol) for the ignition key. Remote is separate rolling code. Always confirm with X431 before buying blanks.

4. How many keys can I program with LAUNCH X431 IMMO Plus?

Up to 8 keys can be stored in the CAS4 memory. You can add or delete keys freely.

5. Why does the tool say "Unsupported CAS Version"?

Your X431 may need an update. Or the CAS module is CAS4+ which is not fully supported via OBD on older software. Update to latest IMMO Plus database.

6. Can I use this tool for other BMW modules (FRM, DME)?

Yes – X431 IMMO Plus also handles FRM, DME reset, and other immobilizer tasks. It is not limited to CAS.

7. What if I brick the CAS module?

If write process is interrupted, the CAS may become dead. A professional repair shop can resolder and reflash via JTAG. Best to use a battery charger and stable network.

Final Verdict

Using LAUNCH X431 IMMO Plus for BMW CAS4 “All Keys Lost” is a cost-effective, relatively safe method when done correctly. Always confirm your CAS generation, stabilize vehicle power, and follow the step-by-step exactly. For DIY enthusiasts, it’s a powerful solution – for difficult cases, consider experts.

Published: May 28, 2026

About the Author: Erwin Salarda is an Automotive Service Equipment Technician based in the Philippines, specializing in computerized automotive diagnostic equipment such as OBD scanners and wheel alignment systems. Since 2012, he has been actively providing technical support, equipment installation, troubleshooting, after-sales service, and hands-on training for automotive service equipment used by workshops and automotive professionals.

He has received specialized training from international suppliers and manufacturers, including Launch X431 in Shenzhen, China, Lawrence Engineering Company in Guangzhou, China, and 3Excel Wheel Alignment Company in Shenzhen, China. Through these trainings, he developed advanced expertise in automotive diagnostics, calibration, wheel alignment systems, and computerized automotive service technologies.

Erwin Salarda provides professional after-sales support and technical training for automotive diagnostic and wheel alignment equipment, helping clients maximize the performance and proper use of their tools and systems. His experience covers equipment setup, software updates, calibration procedures, troubleshooting, and operational guidance for automotive workshops and service centers.

With more than a decade of experience in the automotive equipment industry, Erwin Salarda continues to support automotive businesses by delivering reliable technical expertise and practical training solutions.

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