Mercedes AdBlue Fault Resolved

You’re driving as usual when suddenly, not one but two dashboard lights come on; one for the engine, the other for the AdBlue system. You get home safely and jump online: “Why is my Mercedes AdBlue warning on?”, “Can I drive with it?”, “Do I need a Mercedes dealer to fix it?”

Here’s the truth: yes, the issue needs sorting, but no, you don’t need to book in with a dealership. At Ystrad Service Centre in Ystrad Mynach near Cardiff, we’re highly experienced in handling Mercedes AdBlue faults and delivering prompt, accurate fixes, often on the same day.

Take this recent example. A Mercedes M-Class arrived at our workshop with both warning lights triggered. By the end of the visit, not only had we resolved the underlying fault, but we’d also completed a full Mercedes B1 Service, thanks to our focused diagnostic process and skilled technicians.

Here’s how we identified the issue and delivered a same-day solution, all while maintaining our high standards.

What We Found When the Vehicle Arrived

The M-Class had initially been booked in for its routine Mercedes B1 Service. But shortly before the appointment, the owner contacted us to report something new: two warnings had appeared.  One indicating an engine issue, the other pointing to a Mercedes AdBlue fault.

These alerts aren’t just for show. They’re early indicators that something’s not functioning as it should. Left unchecked, they can lead to higher emissions, reduced performance, or worse, a vehicle that won’t start.

While not all Mercedes AdBlue issues are serious, this one looked more complex from the start. Fortunately, the fault had emerged just ahead of the scheduled booking, so we were able to address both problems in a single visit, saving time and hassle for the customer.

In addition to the planned B1 service, we launched straight into a thorough diagnostic process to uncover what was happening. No hunches. No guesswork.

Getting to the Bottom of the Problem

With the B1 service underway, we connected the M-Class to our diagnostic equipment and ran a full system scan. It wasn’t long before two critical fault codes appeared:

• 205B98 – AdBlue temperature sensor malfunction
  
• 205C15 – Sensor short to ground
  

While these codes won’t mean much to most drivers, they gave us a very clear lead. The AdBlue system was reading temperatures that were much too low, and the system also detected an electrical grounding fault.

Initially, this suggested an internal problem with the tank or sensors, but we don’t rely on surface-level symptoms. We delved deeper.

Our technicians performed a detailed check of the wiring and input signals throughout the system. Everything came back within expected parameters. No physical faults. No visible sensor failures.

That’s when it became clear: the issue wasn’t mechanical; it was software-based.

Something wasn’t right with the AdBlue software, and it was leading to inaccurate sensor readings. This, in turn, created incorrect signals, which triggered the fault codes. The fault codes guided our technicians to the area of the issue, and our thorough technical evaluation found the root cause. It’s the kind of problem that’s easily overlooked unless your team follows a disciplined, structured diagnosis process. We do; and that meant we avoided unnecessary (and expensive) work to our customer’s car. 

What is AdBlue? What Does It Do?

AdBlue is a clear, non-toxic fluid made from purified water and urea. It has its own dedicated tank, which often has a blue cap beside the fuel filler, under the bonnet, or in the boot.

In diesel engines, just like the Mercedes M-Class we cared for, AdBlue is essential. The AdBlue fluid is injected into the exhaust system to neutralise nitrogen oxide emissions, converting them into nitrogen and water vapour. This process helps vehicles meet tough Euro 6 emissions regulations; a legal requirement for most diesel vehicles created after 2015.

When the AdBlue system runs low or experiences a problem, it will usually trigger a warning light. If that’s the situation you’re in, or if you’re concerned about your Mercedes AdBlue levels, give our friendly team a call on 01443 540460.

The Repair: Correcting the Software, Not Replacing Parts

Once we had confirmation that the issue was software-related, we contacted the customer to explain what we’d found. We walked them through the fault codes, the real cause, and the fix required. No jargon, no upselling, just honest technical advice.

At Ystrad Service Centre, we never proceed with repairs without full client approval. Once the customer gave us the green light, we installed the relevant software update to the vehicle’s control unit. This corrected the false data readings and restored the AdBlue system to full working order, without swapping out any physical components.

After completing the update, we performed a full suite of post-repair tests. The system passed with flying colours. No error codes, no warning lights, no lingering issues.

With the Mercedes AdBlue fault sorted, we carried on with the scheduled Mercedes B1 Service, using only high-grade parts that meet or exceed OEM (1) specifications. The filters, fluids, and checks were all carried out to manufacturer standards, and as with all our work, the job came backed by our 12-month or 10,000-mile parts and labour guarantee.

One Visit. Job Done. Back on the Road.

If you’re in Caephilliy, Cardiff or the surrounding areas and your Mercedes is showing an AdBlue warning light or is due for its Mercedes B1 Service, Ystrad Service Centre is here to help.

As your local independent Mercedes specialist, we deliver dealer-level technical accuracy with the approachable service of a trusted neighbourhood garage. Every diagnosis and repair is handled with precision, care, and transparency.

Why Mercedes owners in Cardiff choose Ystrad Service Centre:

✅ Same-day resolution where possible

✅ Free courtesy car to keep you mobile

✅ Highly skilled technicians using dealer-level diagnostic tools

✅ OEM-quality parts used for all servicing and repairs

✅ All work backed by a 12-month or 10,000-mile parts and labour guarantee

✅ Clear communication, and always full approval before work begins

We regularly help local Mercedes drivers with AdBlue issues and B1 services, both quickly and professionally.

⭐ We have a {{average-rating}} star Google rating from {{review-count}} satisfied customers.

📞 Call us today on 01443 540460 to book your Mercedes in at Ystrad Service Centre, Ystrad Mynach.

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(1) OEM stands for Original Equipment Manufacturer. When it comes to vehicle parts, this means the parts are made by the same company that produced the original parts for the vehicle manufacturer