ABS Light Stays On in a Mercedes A-Class W177? Likely a Wheel Speed Sensor

If your Mercedes-Benz A-Class (W177) is showing the ABS light permanently, it’s not something to ignore. In my experience, a constant ABS warning on this generation most often comes down to a wheel speed sensor fault — usually the sensor itself or its wiring. That’s good news, because in most cases the sensor is a separate part from the wheel hub and can be replaced on its own. But I always recommend reading the fault codes from the ABS control module first, because the codes will tell you whether the problem is front-left, front-right, rear-left, or rear-right, and whether it’s a signal error or an electrical circuit issue.

Which parts should you check first?

Start with the wheel speed sensor indicated by the diagnostic scan. On most W177 models, the ABS sensor sits in the wheel bearing/hub area, and the sensor itself is available separately, so you don’t necessarily need to replace the whole hub. Before ordering a new sensor, remove the connector and inspect the wiring harness for chafing, corrosion, or signs of impact. A broken wire or a loose connector can cause the same warning as a failed sensor. Also check the reluctor ring behind the hub for cracks or missing teeth, especially if the sensor was disturbed recently. If the sensor and wiring look good, then the fault could be in the ABS module, but that is less common than a simple sensor failure.

Is it safe to keep driving?

Yes and no. The ABS warning light means the anti-lock system is disabled, and on many W177 models the electronic stability control (ESP) will also shut down to keep the systems from conflicting. Your normal hydraulic brakes will still work, so you can brake in an emergency, but the wheels may lock up on slippery surfaces, and the car may not be able to maintain stability in a skid. That’s a real difference, especially in rain or snow. I’d say it’s safe enough to drive gently to a shop or home, but I wouldn’t take a longer trip. Have the wheel speed sensor fault diagnosed and repaired as soon as possible — the longer you wait, the more likely it is that a small issue (like a corroded connector) will become a larger one.

This material is AI-generated and for informational purposes only — it does not replace an in-person diagnosis by a qualified mechanic.