Immobiliser Repair: BMW CAS, Mercedes EIS/EZS & Transponder Modules
Summary: Most immobiliser faults reach us in one of three states — the engine cranks but will not fire, the car no longer recognises a key that worked the day before, or the security light sits flashing on the dash. We bench test and repair the immobiliser box and transponder for almost all makes and models, including BMW CAS2 and CAS3, Mercedes EIS/EZS with the steering lock and key, and BMW and MINI footwell modules. Your own unit comes back with its coding intact and a Lifetime warranty.
What does a car immobiliser actually do?
Two components do the work, and they sit apart from each other. Inside your key is a transponder chip with no battery of its own — a coil ring around the ignition barrel powers it by induction the moment the key goes in. The chip replies with a code, and the immobiliser control module, often called the transponder box, checks it against the code it holds.
Only when the two agree does the module release the rest of the starting sequence: an enable signal to the starter, then a fresh random code passed to the engine control unit — the DME or DDE on a BMW — which must match the code stored there before fuel injection is released.
That two-stage handshake is why the fault can sit in four different places: the chip in the key, the coil at the barrel, the immobiliser module itself, or the link between that module and the engine ECU. On Mercedes-Benz the Electronic Ignition Switch (EIS, also called the EZS) performs the same authentication and additionally releases the Electronic Steering Lock.
What are the symptoms of a faulty immobiliser?
The symptoms are specific enough to be diagnostic, and they are not all simply “it will not start”.
- Cranks but will not fire — the starter turns the engine normally, but fuel injection is never released.
- No crank at all — where the immobiliser gates the starter feed, nothing happens when you turn the key or press the button.
- Starts, then cuts out after a second — the engine fires before the code check completes, and is shut down when it fails.
- Key not recognised — a dash message, or a key that worked yesterday and does nothing today.
- A security or immobiliser light that flashes or stays lit instead of extinguishing after the check.
- Stored immobiliser fault codes, or no communication with the immobiliser module on a diagnostic tool.
- Central locking, alarm or steering lock behaving erratically, because the same module usually controls them.
Rule out the two cheapest causes before anything else: a flat key battery, and a vehicle battery sitting below the voltage the security modules need. Both mimic module failure convincingly.
BMW CAS2 and CAS3 immobiliser faults
BMW’s Car Access System combines key authentication, immobiliser control and ignition release in a single module, and it is one of the highest-volume immobiliser repairs we handle. On CAS2 cars such as the E87 1 Series, the module sits on the driver’s side behind the storage pocket, with a separate transponder ring at the key barrel; CAS3 units are typically found under the steering column.
The failure pattern is consistent — stored immobiliser fault codes, no communication with the CAS module on a diagnostic tool, a key the car will not accept, and a vehicle that will not start. Most workshops recommend replacement because they cannot repair the unit at board level, and replacement is not straightforward: a used module is locked to the car it came out of, and a new one is a main-dealer part that has to be programmed and locked to your vehicle.
We test and repair both generations — BMW CAS2 immobiliser module repair and BMW CAS3 immobiliser module repair. Repaired units keep their original programming and are returned ready to refit.
Mercedes EIS / EZS ignition switch, steering lock and key faults
On Mercedes-Benz the immobiliser lives inside the Electronic Ignition Switch — the EIS, or EZS — which authenticates the key, releases the immobiliser, disengages the Electronic Steering Lock (ESL) and also governs central locking and the alarm. It is fitted right across the range, from the W168 A-Class through the W203, W204, W211, W219, W220 and W221 to the W639 Vito and Viano.
What kills it is usually a voltage event rather than age. Jump starting with the key still inserted, a power surge, or running the car on a battery that has dropped too low can corrupt the data held in the module — and once that has happened, the unit cannot be re-flashed through Mercedes STAR diagnostics. That is the point at which most garages stop.
Because the EIS, the ESL and the key work as a set, we ask for all three so they can be tested individually and then in circuit together. Full detail is on our Mercedes EIS/EZS immobiliser and key module repair page.
Not every “immobiliser” fault is one. A large share of the BMW and MINI work in this category turns out to be the Footwell Module (FRM), which sits in the footwell and controls headlights, tail lights, brake lights, indicators, interior lighting, windows, central locking and wipers.
It reads as a security fault because of what it takes down: central locking stops responding, lights behave erratically or not at all, warning messages appear, and a diagnostic tool reports no communication with the FRM. The trigger is usually electrical rather than mechanical — a flat battery, a jump start or a battery change can corrupt the module’s stored data, while water ingress and corrosion do the same over a longer period.
The same module and the same failure recur across a defined set of cars: the BMW 1 Series (2006–2011), 3 Series (2004–2013), X1 (2008–2015), X5 (2009–2015) and Z4 (2008–2015), and on the MINI side the Cooper (2006–2015), Clubman (2006–2014) and Cabrio (2007–2015). Our BMW and MINI FRM3 footwell module repair service covers the whole family.
Repair or replace? Why a used or new module is rarely the shortcut
Replacement sounds like the simple answer and rarely is. A used module carries the coding of the car it was removed from and is locked to it, so it cannot simply be fitted to yours. A new module is a main-dealer part on many of these vehicles, supplied blank, and has to be programmed and locked to your car and matched to your existing keys — or the keys replaced alongside it. On older models, availability is an obstacle in its own right.
Repairing the unit you already have avoids all of that. The original coding and key matching stay with the module, so in most cases it goes back on the car without programming. Repair also lets us address the component that actually failed and reinforce the weak point, rather than fitting an identical part with the identical flaw waiting inside it.
And if required, we can decode the immobiliser to de-activate it.
How our mail-in immobiliser repair works
This is a mail-in repair-and-return service, so the vehicle stays where it is and only the unit travels.
Tell us the vehicle, the symptoms and, where you have it, the part number on the module, and we will confirm whether the fault is likely to sit in that unit before you send anything. Remove the module — on a Mercedes, send the EIS, the ESL and the key together — and post it in.
On the bench we power the unit up and simulate the vehicle around it, so the fault is confirmed in operation rather than inferred from a stored code. Repairs are carried out at circuit-board level by our in-house electronics engineers in an ESD-safe, cleanroom-standard workshop, with the programming data backed up and restored so the unit comes back matched to your car and your existing keys. Every unit is retested before it leaves us, and immobiliser repairs are returned with a Lifetime warranty.
We do not supply removal or fitting instructions, and we recommend that removal and refitting are carried out by a competent technician. To get started, send us the details through our repair form, or contact us if you are not yet sure which unit has failed.
Immobiliser repair FAQs
Can a car immobiliser be repaired?
In most cases, yes. The immobiliser box and transponder are circuit assemblies, and the usual failures — corrupted stored data, failed components, damage following a voltage event — can be repaired at board level. Repaired units keep their original coding, so they normally refit without programming.
What are the signs of a faulty immobiliser?
The engine cranks but will not fire, or will not crank at all; the car refuses to recognise a key; it starts and cuts out after a second; the security light stays on or flashes; and a diagnostic tool reports immobiliser fault codes or no communication with the module.
Can I fit a second-hand immobiliser module?
No. Immobiliser modules are coded and locked to the vehicle they were fitted to, so a used unit from another car will not work in yours. A new module is a main-dealer part on many vehicles and still has to be programmed and locked to your car.
Will a repaired module need coding or programming?
Usually not. We back up the programming data before remanufacturing and restore it afterwards, so the unit is returned matched to your vehicle and your existing keys and goes back on plug-and-play.
Can a flat battery or a jump start cause an immobiliser fault?
Yes, and it is one of the most common triggers we see. Jump starting with the key inserted, a power surge, or running on a battery that has dropped too low can corrupt the data held in the module — and a Mercedes EIS cannot then be re-flashed through STAR diagnostics.
Which makes and models do you cover?
We bench test and repair immobiliser and transponder units for almost all makes and models, with the highest volume through BMW CAS2 and CAS3, Mercedes EIS/EZS with the ESL and key, and BMW and MINI footwell modules. If your unit is not listed, send us the part number and we will confirm.
Final thoughts
An immobiliser fault is one of the few electronic failures that leaves a mechanically healthy car completely immobile — which is why it so often ends in a quote to replace a module that was repairable all along. The unit already on your car holds the coding that matches your keys; in most cases the sensible route is to fix it and keep it, rather than start again with a blank part and a programming appointment. Send us the part number and the symptoms and we will tell you what has failed before you commit to anything.
