Which Fault Codes the Mercedes W204 ABS Module Sets and Why

The ABS control module on the Mercedes W204 C-Class does not fail silently. When it detects a problem — whether in itself, in a wheel speed sensor, or in the hydraulic pump circuit — it logs a specific fault code and illuminates the ABS warning light, often together with ESP and BAS warnings. Understanding what each code is actually measuring is the difference between a ten-minute sensor replacement and an unnecessary module swap.

What the W204 ABS Module Actually Controls

The W204 uses a Bosch ESP 8 or ESP 9 hydraulic control unit depending on build date and market. The control module is integrated into the hydraulic unit — they are one assembly. It reads four wheel speed sensors, monitors brake pressure, communicates with the EZS, the ESP control logic, and the instrument cluster via CAN. When the module detects a fault, it disables ABS and ESP individually or together depending on the severity.

This integration matters for diagnosis: a fault inside the hydraulic pump solenoids is inside the same housing as the control electronics. A workshop that replaces only the electronic part of the assembly without checking the hydraulic side will sometimes find the fault returns immediately.

Fault Codes Specific to the W204 ABS System

The following codes appear in Mercedes diagnostic systems (XENTRY/DAS) on the W204. The same codes can appear in third-party tools including Autel and iCarsoft, though the description text may differ.

C100400 — Left Front Wheel Speed Sensor: Signal Implausible

This code means the module received a speed signal from the left front sensor that did not match the other three channels. It does not immediately mean the sensor is dead — a corroded connector at the sensor plug, a damaged tone ring, or a sensor gap that has opened up due to worn hub bearing will all produce this code. Confirm by checking live wheel speed data at all four corners while rolling at walking pace. For comparison, see Which Modules the Mercedes W204 C-Class Shares With the W212 and W207.

C100800 — Right Front Wheel Speed Sensor: Signal Implausible

Identical logic to C100400, applied to the right front. Both front sensors on the W204 are active Hall-effect types running on a 12 V supply from the module. Check supply voltage at the sensor connector (should be 10.5–12.5 V with ignition on) and signal output (typically a square wave between 0 V and supply voltage). No supply voltage points toward the module output, not the sensor itself.

C101200 — Left Rear Wheel Speed Sensor: Signal Implausible

Rear sensor faults on the W204 are disproportionately common in cars that have seen road salt. The rear sensor connectors sit low and trap moisture. Inspect the connector body and the sensor cable for corrosion or chafing before condemning the sensor. Resistance checks do not work on active sensors — you need to check supply and signal live.

C101600 — Right Rear Wheel Speed Sensor: Signal Implausible

Same diagnosis path as C101200. If both rear sensor codes appear simultaneously, check the ground path for the rear sensor supply before assuming both sensors have failed together — a shared ground point corroding through is a known W204 occurrence.

C249F00 — ABS Control Module: Internal Fault

This code indicates the module has detected a fault in its own internal logic or memory. It cannot be cleared by a sensor replacement. If it appears alongside sensor codes, address the sensors first and retest — a sensor pulling supply voltage to ground can trigger an internal fault code. If C249F00 stands alone and returns immediately after clearing, the module itself is the primary suspect.

C006300 — Pump Motor Relay: Open Circuit or Short

The hydraulic pump inside the ABS unit is driven by a relay within the module assembly. This code appears when the relay circuit is open or when the pump draws no current during a self-test. A seized pump motor or an open relay winding will both set it. The pump motor can sometimes be tested by listening for it during the ABS self-test at startup — a healthy pump makes a brief audible buzz as you release the brake and pull away above 5 km/h.

C121A00 — ESP Hydraulic Pressure Sensor: Implausible Signal

The brake pressure sensor sits inside the hydraulic unit and reports master cylinder pressure to the ESP logic. If it drifts or fails, the module cannot calculate the correct pressure demand for stability interventions. This code does not appear on all W204 variants — the standard ABS-only system does not have this sensor; only cars with full ESP use it.

Codes That Look Like ABS Faults But Are Not

Two codes regularly mislead technicians working on the W204. First, a low battery voltage condition — anything below 11.2 V at the module supply — will cause multiple wheel speed channels to read erratically and log implausible signal codes across all four corners simultaneously. If all four sensor codes appear at once, check charging system voltage before touching anything else. A related diagnostic guide is Buying a Used Mercedes W204 C-Class SAM Module: What to Check.

Second, a U010087 — Lost Communication With ABS Module — appearing in the instrument cluster or EZS fault memory does not mean the ABS module is dead. It means the CAN connection to the module was interrupted. Check the CAN wiring at the module connector and at the CAN gateway before assuming internal module failure.

OEM Part Numbers for the W204 ABS Module

The W204 ABS/ESP unit changed across production, so the part number must match the build date, engine variant, and whether the car has 4MATIC or rear-wheel drive. The following references cover the mainstream RWD W204 C-Class:

A0044315812 — ESP 8 unit, early W204 (2007–2009), six-cylinder variants
A0054312912 — ESP 8 unit, four-cylinder petrol variants, 2007–2010
A2044316512 — ESP 9 unit, post-2010 facelift, diesel and petrol four-cylinder
A2044317112 — ESP 9 unit, 2010–2012 C63 AMG and V6 petrol variants
A0044316312 — 4MATIC all-wheel drive variant, 2008–2011

Part number matching on the W204 is not optional. The coding inside the module references the vehicle type, axle configuration, and tyre size variant stored in the EZS. Fitting a module with a different last three digits of the part number will result in ESP errors and possible limp mode even if the unit physically connects. Always confirm the full A-number from the original unit before sourcing a replacement.

Affected Vehicles and Build Dates

The W204 C-Class ran from model year 2007 to 2014, but the 2007–2012 range covers the majority of used examples in circulation. The Bosch ESP 8 unit was standard until approximately late 2009, with ESP 9 appearing on facelifted cars from 2011 onward. Transition-year cars (2010 build) may carry either unit depending on production month.

Platform-sharing vehicles that use closely related ABS hardware include the W212 E-Class (2009–2012) and the W207 E-Class Coupé, though the part numbers do not cross over without confirming the full reference — the vehicle configuration stored in each module differs.

Before Replacing the Module

A used ABS module for the W204 can be fitted, but it requires coding via XENTRY or equivalent Mercedes-capable software to marry the module to the vehicle’s configuration data. The module stores wheel size, vehicle type, and ABS intervention thresholds. An uncoded replacement will run but will log configuration faults immediately.

When sourcing a used unit, confirm the full part number from the donor vehicle and ask for the build date. Tested units from GiantParts are checked for internal relay operation and sensor supply output before shipping, which saves a fitment only to discover the pump relay was already open in the donor car. For additional context, read Mercedes W204 SAM Module Faults: Symptoms and Repair.

FAQ

Can a Mercedes W204 ABS module be repaired rather than replaced?

Board-level repair is possible for some internal faults — particularly failed relay contacts and cracked solder joints on the pump driver circuit. However, if the hydraulic block itself has seized valves or a dead pump motor, the electronics cannot be separated from the hydraulic unit without specialist equipment. A tested used OEM assembly is usually more cost-effective than a partial repair on the electronic section alone.

Does a replacement W204 ABS module need coding?

Yes. The W204 ABS module stores vehicle-specific configuration including axle type, tyre rolling circumference, and ESP intervention thresholds. Without XENTRY SCN coding or equivalent variant coding, the replacement unit will operate in a restricted mode and log configuration faults. This is not a procedure that can be skipped or bypassed.

Why do all four wheel speed sensor codes appear at the same time on my W204?

Simultaneous faults on all four channels almost always point to a shared cause — low battery voltage, a failing alternator, or a corroded supply or ground wire to the ABS module rather than four sensors failing at once. Check charging voltage first and inspect the module connector for corrosion before any further diagnosis.

Will a W204 ABS module from a diesel car work in a petrol W204?

Not without recoding, and in some cases not at all. The ABS module stores engine variant data and the ESP calibration differs between diesel and petrol applications because braking behaviour and engine braking characteristics differ. The safest approach is to source a unit from the same engine family and have it coded to match the recipient vehicle’s configuration.

Is the ABS warning light alone enough to confirm the module has failed?

No. The ABS warning light on the W204 illuminates any time the module detects a fault it cannot handle — including a single faulty wheel speed sensor, a wiring issue, or low battery voltage. The light itself proves nothing about the module’s condition. Fault code reading followed by live data testing of the sensor channels is the only way to establish whether the module or its inputs are at fault.

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