Mercedes EIS Repair – EZS Immobiliser Key Module – ESL + KEY Repair Service

The table below turns what your car is doing into the part to suspect and what to send, so the right repair is booked before anything comes out of the dashboard. Other makes and immobiliser types are covered from our immobiliser repair hub.

Which part has failed? Match what your Mercedes is doing to the unit

Start with what happens when the key goes in. Each behaviour points to a different member of the set, and the last column is what we need to see to confirm it.

The key-not-recognised message is the one owners most often blame on the key, and sometimes they are right. But if the spare key produces exactly the same message, two keys failing together is far less likely than the EIS that reads them having failed. That one comparison is worth making before anything is removed.

If the steering lock has stuck in the locked position, the car cannot be steered, so it is a recovery job rather than a drive to the garage. Never force a key that will not turn.

EIS, EZS, ESL and the key: what each part does

EIS and EZS are one unit under two names. EIS stands for Electronic Ignition Switch; EZS is the German abbreviation, for Elektronisches Zündschloss, and in Mercedes wiring terms it is component N73. It is the slot your key goes into, and it decides whether the car may start: it checks the key, clears the immobiliser with the engine control unit, and on most models also looks after the central locking and alarm.

The ESL is the steering lock. The Electronic Steering Lock, which some sources call the ELV, is a separate unit on the steering column. It locks the column when the car is parked and releases it only once the EIS has accepted a valid key. A lock that will not release leaves the key unable to turn, however healthy the EIS is.

Not every Mercedes has an ESL. On some models the steering-lock function is handled through the automatic gearbox instead. On those cars there is no ESL to send, and we need only the EIS and the keys.

The key is the third part of the set. It carries the code the EIS is matched to, as well as the remote locking. A key and an EIS that have stopped agreeing produce the same symptoms as either one failing outright.

Why the EIS, ESL and key are tested together

The three parts do not simply work in turn; they vouch for each other. The key identifies itself to the EIS, the EIS instructs the steering lock, the lock reports back, and only when every answer is right is the immobiliser cleared. Break any link and the car shows you the same thing: a key that will not turn, or an engine that will not crank.

That is why one part tested on its own can pass and still be the fault, or fail and be innocent. An EIS sent without its steering lock can behave perfectly on the bench, because what was stopping it on the car was the lock. A key that seems dead may simply be a key the EIS no longer accepts.

So we ask for the whole set. Each part is tested individually, then all three are run together in circuit, as they would be on your car, until the fault is pinned to the unit that actually has it. Where more than one has worn, such as a tired steering lock that has been straining an ageing EIS, we can remanufacture all three and return them as a working set.

Swapping parts on suspicion cannot do this. A second-hand EIS or a new key tests one guess at a time, and each has to be programmed to the car before it can even be tried.

What makes a Mercedes EIS or steering lock fail?

Failures usually trace back to one of three causes:

• Voltage events. A jump-start done with the key left in, a supply surge, or repeated starting on a flat or failing battery can leave the EIS with corrupted stored data.
• Wear. The EIS takes a key insertion on every journey and the steering lock locks and releases the column each time, so both age with use as well as with time.
• Temperature. A dashboard bakes in summer and chills overnight in winter, and cold mornings are typically when an intermittent fault first shows.

Voltage damage is where dealer tooling runs out. Once an EIS has lost its stored data that way, Mercedes STAR diagnostics can neither recover nor re-flash it, and the usual advice becomes a new unit. Our bench work can bring these units back.

A new EIS is rarely the quick fix it sounds. It has to be programmed to the car and matched to the keys, and on many models it is a special-order item, which adds its own delay.

For how these faults tend to build up on the road before the car finally refuses to start, read our guide to Mercedes EZS/EIS ignition switch faults.

Which Mercedes models have the EIS?

The EIS is factory-fitted to almost every Mercedes of its era, and units reach us from right across the range: C-Class, E-Class, S-Class, CLS, CLK, CL, ML, GL, R-Class, SL and SLK. By chassis code, the EIS is fitted to:

W164 / W168 / W169 / W202 / W203 / W204 / W208 / W209 / W210 / W211 / W212 / W215 / W219 / W220 / W221 / W230 / W639

The table lists vehicles we have already had in with EIS faults. It is not a complete list, so if your Mercedes is missing, send the details anyway.

BMW’s equivalent is the CAS module, and we repair both CAS2 and CAS3 units.

How the repair, recoding and return work

1. Book it in

   Complete the repair form with your model, chassis code and what the car does. Tell us whether the spare key behaves the same way; that detail alone often separates a key fault from an EIS fault.

2. Your garage removes the parts

   The EIS and, where fitted, the ESL are taken out and posted to us with every key for the car. We do not publish a removal guide: removal and refitting are a job for a competent technician, and the car waits with them.

3. Data backed up, fault confirmed

   Before any work starts, the programming data in each unit is backed up. On our test bench the EIS is powered up, communicated with and simulated as if it were on your car, then all three parts are run together to confirm where the fault sits.

4. Repaired and recoded to your keys

   The failed part is repaired and the set is recoded to the keys you sent, so nothing needs programming at a dealer. If an EIS is beyond repair, we supply a replacement and program it on our bench, so it needs no further coding when fitted.

5. Retested and posted back

   The finished set is tested again on our in-house Hardware-in-the-Loop rigs, which reproduce the heat, vibration and electrical load of the car. It returns plug and play, self-calibrates once refitted, and carries our Lifetime warranty.

Because the whole service runs by post, it makes no difference where in the UK the car is. The parts come to our workshop and go back to your garage.

Common Mercedes EIS part numbers

These are the part numbers we see most often. The list is not complete and there are many more we have repaired, so if yours is missing, put it on the repair form and we will confirm.

Mercedes EIS repair FAQs