Mercedes W202 C-Class Immobilizer EIS Faults Diagnosis and Repair Guide

The Mercedes W202 C-Class immobilizer system, integrated into the Electronic Ignition Switch (EIS) module, is one of the most misunderstood sources of no-start faults on these cars. When the EIS fails or the immobilizer loses synchronisation with the key transponder, the engine simply will not crank — or it cranks but refuses to fire. The good news is that most of these faults are diagnosable with the right scan tool and a methodical approach, and tested used OEM modules are widely available at a fraction of new dealer prices.

What Exactly Is the W202 Immobilizer System?

Unlike later Mercedes platforms, the W202 uses a relatively straightforward immobilizer architecture. The EIS module reads the transponder chip embedded in the key, then communicates with the ECU (ME 2.1 or HFM depending on engine variant) and the instrument cluster to allow engine start. All three modules must agree on a shared code before fuel injection and ignition are released.

When any part of this chain breaks — a corrupted EIS, a dead key transponder, or a mismatch after ECU replacement — you end up stranded. What makes diagnosis tricky is that the symptoms can look identical whether the fault is the EIS, the ECU, or just a worn key battery. Always start with the simplest checks first.

Common Symptoms of EIS and Immobilizer Failure

Engine cranks normally but will not start — no fuel delivery, no injector pulse
Red immobilizer warning light stays on or flashes continuously on the instrument cluster
Key turns in the ignition but nothing happens — no crank, no accessories
Intermittent no-start that clears after repeated key cycles or leaving the car overnight
Loss of all electrical functions when key is inserted (rare, points to EIS power supply fault)
Remote central locking stops working alongside no-start (EIS controls both on some variants)

Common Error Codes — W202 Immobilizer and EIS

Код на грешката	Описание
B1000	Control unit internal fault — EIS self-check failed
B1001	Transponder communication error — key not recognised
B1004	Immobilizer active — engine start blocked by EIS
P0513	Incorrect immobilizer key — ECU-side transponder mismatch
P1690	Immobilizer signal missing or implausible — no EIS response received by ECU
U1001	CAN communication fault — EIS not responding on the bus

Bear in mind that many generic OBD-II readers will not pull Mercedes-specific B and U codes. You need a Star Diagnosis (DAS/Xentry), iCarsoft MB II, or Autel with Mercedes coverage to read the full EIS fault memory. Generic readers on the OBD port will often return nothing even when the EIS is clearly at fault. For comparison, see Mercedes W202 C-Class Immobilizer EIS Faults Diagnosis and Repair Guide.

Why Do These Modules Fail?

The W202 was built from 1993 to 2000, and most survivors are now 25 to 30 years old. Age alone is enough to cause problems. Specifically:

EEPROM corruption — the small memory chip inside the EIS stores the transponder codes; over time, especially after flat battery events or jump-start voltage spikes, this data corrupts.
Умора на спойката — the ignition switch area experiences vibration and heat cycling every day; solder joints on the EIS PCB crack over time, causing intermittent contact loss.
Проникване на влага — the EIS sits close to the steering column; leaks from the windscreen or sunroof can allow condensation to accumulate and corrode the module contacts.
Key wear — the transponder coil in the key head can fail without any obvious physical damage. Always test a spare key before condemning the EIS.

Diagnostic Steps Before Replacing Anything

Step one: try the spare key. If the car starts with the spare, the problem is a dead transponder in your main key — a locksmith or dealer can recode a new blade for around €50, which is far cheaper than an EIS replacement.

Step two: check the battery. A voltage below 11.8V can cause the EIS to malfunction even if the hardware is fine. Fully charge or replace the battery, then retest.

Step three: connect a Mercedes-compatible scanner and read the EIS fault memory. Log all active and stored codes. If you see U1001 or CAN bus faults, check the EIS power supply fuse (typically fuse 15 in the front fuse box) and ground connections at the steering column before replacing the module.

Step four: if B1001 or P0513 are present and persist with a known-good key, the EIS EEPROM data is very likely corrupted or the module has developed an internal fault. At this point, replacement or specialist remanufacturing is the only path forward. A related diagnostic guide is Mercedes W202 C-Class Transmission Control Unit Faults Repair Guide.

Common OEM Part Numbers — W202 EIS and Immobilizer Components

OEM номер на частта	Приложение
A 202 545 02 08	EIS module — C180/C200 petrol variants (M111 engine)
A 202 545 03 08	EIS module — C220/C230 petrol (M111 2.2/2.3)
A 202 545 04 08	EIS module — C250/C280 six-cylinder (M104)
A 202 545 05 08	EIS module — C220 CDI diesel (OM604/OM611)
A 202 820 00 26	Key transponder assembly — standard W202 key head
A 000 905 30 00	Immobilizer control relay — early W202 auxiliary relay

Always cross-reference with your VIN before ordering. The exact EIS variant fitted depends on engine code, build date and market. A used OEM unit sourced from a matching donor vehicle is the most cost-effective and reliable solution — provided it is properly coded to your car.

Засегнати превозни средства

The EIS and immobilizer faults described here primarily affect the Mercedes W202 C-Class produced between 1993 and 2000, covering all engine variants: C180, C200, C220, C230, C250, C280 petrol and C220 CDI diesel. The early pre-facelift models (1993–1996) use a slightly different immobilizer generation than the post-facelift (1997–2000) cars — always confirm the exact module generation when sourcing a replacement.

The W202 AMG C36 and C43 variants share the same EIS architecture but use different ECU part numbers, so coding requirements differ slightly. Platform derivatives such as the S202 estate (T-Model) are identically affected.

Често задавани въпроси

Can I replace the W202 EIS module myself without dealer coding?

Not easily. The replacement EIS must be programmed with your car’s specific transponder code and synchronised with the ECU and instrument cluster. This requires Mercedes Star Diagnosis or a specialist immobilizer programmer. Some independent workshops with the right equipment can do this at a lower cost than a main dealer. Fitting the module physically is straightforward — coding it is not a DIY job.

Will a used EIS from a scrapped W202 work in my car?

Yes, but only after it is coded to match your vehicle. A used EIS retains the previous car’s transponder data in its EEPROM. A specialist must overwrite this with your data, or alternatively provide you with both the used EIS and a matched key set from the same donor car — which avoids recoding entirely but means changing all your locks. For additional context, read Mercedes W202 C-Class ABS Control Module Failure Diagnosis Guide.

My W202 cranks but will not start — is it definitely the immobilizer?

Not necessarily. A cranking no-start has many possible causes: fuel pump relay, crankshaft sensor, camshaft sensor, or ECU faults can all mimic immobilizer symptoms. The definitive check is to read the injector driver signal with an oscilloscope — if the ECU is not triggering injectors and there are no crank/cam sensor codes, then an immobilizer blockage is very likely the cause.

How much does W202 EIS replacement cost at a workshop?

A tested used OEM EIS module typically costs €80–€150 sourced from a reputable European supplier. Labour and coding at an independent workshop adds €100–€200 depending on location and complexity. Total cost is usually €180–€350, compared to €600–€900 for a new OEM unit with dealer coding — making tested used OEM the obvious choice for a car of this age.

What is the difference between the W202 EIS and the later W203 EIS/EZS?

The W202 uses a first-generation Mercedes immobilizer system where the EIS is a simpler relay-based control unit integrated into the ignition switch assembly. The W203 introduced the more sophisticated EZS (Electronic Ignition Switch) with CAN bus integration, infrared key communication and significantly more complex coding requirements. Parts are not interchangeable between the two platforms.

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