When the Mercedes W168 A-Class Ignition Switch Module Fails

The ignition switch module on the Mercedes W168 A-Class is a small but critical electrical component mounted on the steering column. It carries the switched live circuits for the engine management, instrument cluster, and EWS immobilizer communication. When its internal contacts corrode or the plastic housing cracks, the result is anything from intermittent start failures to a completely unresponsive dashboard. This is not a mechanical ignition barrel problem — it is an electrical module failure, and the two are easily confused during diagnosis.

What the Ignition Switch Module Actually Does

The W168 A-Class uses a two-part ignition assembly. The mechanical lock cylinder handles the key and the steering lock. The electrical ignition switch module sits behind it on the column and translates key position into switched electrical signals — accessory, ignition-on, and start. Position II energises the main relay circuit for the ECU and instrument cluster. Position III sends the starter signal. If the module fails, those signals do not reach their destinations regardless of what the key and lock cylinder are doing. Owners often replace the lock barrel or the EIS module unnecessarily when the simpler switch module is the actual fault.

How the W168 Ignition Switch Fails in Practice

The failure pattern on high-mileage W168 cars built between 1997 and 2004 follows a recognisable sequence. Early on, the dashboard may flicker briefly when the key is returned from position III to position II after a start attempt. At this stage the car usually starts normally but the cluster momentarily loses power. As wear progresses, the start signal becomes intermittent — the starter motor does not engage on the first turn of the key, but works on the second or third attempt. Eventually the car may crank with no other electrical systems active, or sit completely silent with no instrument cluster response at all.

Heat cycles accelerate contact wear. The W168 engine bay is compact and the steering column sits close to sources of heat. Moisture ingress through the footwell — a known weakness on older A-Class cars — can reach the base of the column and attack the switch module contacts from below. For comparison, see When the Mercedes W168 A-Class ECU Loses Its Engine Start Signal.

Fault Codes Associated With This Failure

A STAR diagnostic session or compatible third-party tool will often log faults in the engine control unit and occasionally in the instrument cluster when the ignition switch module is failing intermittently. The following codes appear most frequently:

Code	Description
P0615	Starter relay control circuit — switch signal not reaching ECU cleanly
P0685	ECM / PCM power relay control circuit open — ignition supply intermittent
B1000	Control unit supply voltage fault — cluster loses ignition feed
B1001	Ignition switch position signal implausible
U0100	Lost communication with ECU — often a downstream effect of power loss

Not all of these codes appear together, and some only log as intermittent stored faults rather than current faults. A clean scan does not rule out a failing ignition switch module if the symptoms match.

Confirming the Fault Before Ordering Parts

Before replacing the ignition switch module, spend a few minutes at the fuse box. The W168 has a main fuse box in the engine bay and a secondary unit in the driver’s footwell. Check fuse positions 1, 3, and 15 on the interior board — these carry the ignition-switched feeds most directly affected by switch module failure. If fuses are intact, use a multimeter to back-probe the column connector while an assistant operates the key. You should see a clean transition from zero to battery voltage at the switched outputs when moving from position I to position II. Any hesitation, dip, or absence of voltage at position II with a known-good battery confirms the switch module rather than the wiring or relay.

Also worth checking: the multi-function steering column stalk cluster. On some W168 variants, a faulty stalk unit creates voltage spikes that damage the ignition switch contacts over time. If the stalks feel loose or produce erratic wiper and indicator behaviour, inspect them alongside the switch module. A related diagnostic guide is Mercedes W168 A-Class Instrument Cluster Faults Common Failures Repair.

OEM Part Numbers for the W168 Ignition Switch Module

Mercedes revised the ignition switch module during W168 production. The early 1997–2001 cars used a different connector arrangement from the 2001–2004 facelifted models. Matching the exact OEM number is important because a late-model switch will not plug into an early-model column without modification.

A 004 545 18 04 — early W168 (1997–2001), standard ignition switch module
A 004 545 22 04 — revised variant introduced mid-production, fits 1999–2001 late build
A 168 545 00 04 — W168 facelift (2001–2004), updated connector body
A 168 545 01 04 — facelift variant with amended contact pack, most common replacement

Always cross-reference the part number printed on the original module before ordering. Suppliers sometimes list these interchangeably, but the connector housings are not identical across the production run. A used OEM module with a matching number is preferable to a generic aftermarket replacement because the contact geometry and switching thresholds are engineered to Mercedes tolerances.

Affected Vehicles

The ignition switch module described here applies to the W168 A-Class produced from 1997 through to the end of production in 2004, covering the A140, A160, A170 CDI, A190, and A210 variants. Left-hand-drive and right-hand-drive configurations share the same module design but may have differently routed column looms. The switch module is not shared with the W169 A-Class, which uses an entirely different EIS-based ignition system.

Frequently Asked Questions

Can a failing ignition switch module on the W168 drain the battery?

Not typically in the same way a SAM module or gateway does. The ignition switch module is passive when the key is removed and does not draw a quiescent current large enough to flatten a battery overnight. However, if the switch is failing at position II and keeps certain relays partially energised, a small parasitic draw is possible. A standard quiescent current test will confirm this quickly.

Does the replacement W168 ignition switch module need coding?

No. The W168 ignition switch module is not a coded component. It does not communicate with the EWS immobilizer directly — that function is handled elsewhere in the column and by the separate EWS control unit. Replacement is a straight swap once the correct OEM number is confirmed. No STAR adaptation or coding session is required. For additional context, read Mercedes W168 A-Class ABS Control Module Failure Diagnosis Guide.

Is it safe to drive a W168 with an intermittent ignition switch fault?

It depends on the stage of failure. Early-stage flickering is a reliability issue rather than an immediate safety risk. However, if the switch is cutting the ignition feed to the ECU while driving, the engine can cut out without warning. At that point the car should not be driven until the switch module is replaced.

How long does the ignition switch module replacement take on a W168?

With the correct tools, the job takes around 45 minutes to one hour. The steering column lower shrouds must be removed to access the switch module housing. The connector release can be awkward on corroded examples. No airbag deactivation is required unless the technician is also working on the stalk cluster in the same session.

Can a used OEM ignition switch module be reused safely?

Yes, provided it comes from a low-mileage donor car and the contact surfaces inside the module show no pitting or carbon tracking. A tested used OEM module with a matching part number is a practical choice for a W168 repair. Inspect the connector pins for corrosion before fitting, and apply a light coating of contact grease to the plug before reassembly.

Find tested OEM electronic parts and modules at:
https://giantparts.eu/

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