Open Monday-Thursday 8.30am-17.00pm | Friday 8am-16.30pm 091 792474

Mercedes Service Galway: How to fix a Mercedes-Benz C220 with AdBlue System Failure. (Error code: P120300 – Fixed)

Did you know that the Mercedes C class was voted the overall World Car of the Year in 2015?  Impressive, right? That’s quite an accomplishment. There are 72 jurors, including some of the most prominent names in motoring.

As a Mercedes C Class owner, you know that there’s a certain ‘something’ that keeps you loyal to the premium brand. It should go without saying that all vehicles, even premium German brands, require regular service and maintenance to keep them in top form.

So, when you encounter an issue with such a premium car, it’s important to get it fixed as soon as you can.

Issues we found

As soon as Mr K brought his Mercedes C220 to our Mercedes garage Galway with a problem, our Mercedes specialists immediately began to investigate and diagnose the issue. Here at Donnellan Auto Services, we believe in making sure our clients get the best performance from their vehicles. We do this by completing a full investigation to uncover the root cause of the problem.

In this case, the dashboard was highlighting a fault with the Engine Management Light (EML), Engine Control Unit (ECU) and also, there was an AdBlue warning showing. Our next step was to get to the root of the problem. Read on to find out how we went about this……

Diagnosing the problem

By using our 15 step diagnostic evaluation, we make sure that we are able to identify the root cause of the issue and are able to provide a first-time fix for the client. As your local independent Mercedes garage, our team conforms to recommended manufacturer guidelines, protecting the integrity of your vehicle exactly as Mercedes intended.

So when Mr K told us that he had faults highlighted on the dashboard, we knew that we could investigate the problem for the client and get him back on the road in no time. 

According to the client, he had filled his Mercedes AdBlue but had overfilled it. What Mr K didn’t know is that overfilling the AdBlue system can cause further damage. It is crucial for the sophisticated system to have everything operating at the correct level.

Our team of Mercedes specialists followed our 15 step diagnostic evaluation and visually inspected the vehicle but nothing immediately stood out. So, we performed a full assessment of the vehicle’s serial data.

Fixing the problem

After accessing the vehicle serial data, we found the following fault code:

P140300 – The control unit AdBlue has a malfunction

Armed with the serial data, our Mercedes Galway specialist technicians accessed the AdBlue pump and started to check the system’s wiring. They found that there was no power going into the AdBlue module, so they began stripping back the wiring to locate the defect.

Upon further inspection, it was discovered that the main loom had completely rubbed through, resulting in water contamination and some serious corrosion to the power wires. In this case, we were able to perform the wiring replacement and re-insulation onsite at Donnellan Auto Services.

As the technicians were working on the AdBlue system, they also noticed a problem with the level sensor. The client had already told us that he had overfilled the system, which shouldn’t happen when working with an ultrasonic sensor system. Fortunately, our knowledge and experience have taught us that if you overfill the AdBlue system, the ultrasonic sensor is unable to detect the level and thus misleads the vehicle’s electronics.

After draining the AdBlue tank system, we refilled the tank to the correct level. Once the Mercedes C220’s AdBlue level was correct, we took it for a test drive to confirm that the Engine Management Warning light had disappeared and it was back to being reliable and road safe.

In less than 24 hours, the client was able to get his car back. The first-time fix he received from Fergies saved him from the stress and money of repeated repairs.  We explained everything to Mr K in detail so that he understood what was going on and that there wouldn’t be any unnecessary costs.


The moment Mr K brought his Mercedes C220 to our Galway location, our Mercedes specialists got to work on a complete diagnostic evaluation, in order to pinpoint the fault and get him back on the road as quickly and safely as possible.

Our investigation revealed an issue with the AdBlue system.

As our team has access to all the Mercedes manufacturer information and years of experience in servicing and maintaining German premium vehicles, we were able to diagnose the problem and get the C220 back to the client within 24 hours.

At Donnellan Auto Services, we believe in a transparent approach to our business, and we make certain that our clients are fully involved in the process of maintaining their vehicles and are never surprised with unexpected charges.

We will take care of your Mercedes car for you, giving you peace of mind and great value. Contact Donnellan Auto Services today.

Contact us at Donnellan Auto Services for your Mercedes service Galway on 091 792474.

Call Now 091 792474Save on Mercedes servicing