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Mercedes C220 AdBlue Fault

According to the Fleet News Awards, in 2019, the Mercedes C Class was voted premium car of the year. And Mercedes has advised that although the premium vehicle looks exactly as it did before, there have been more than 6000 changes to the C Class, making it more efficient than ever before. 

If you are a C Class owner, you may have noticed that there have been upgrades to ensure that Mercedes stays top of the premium car pile. Whilst Mercedes is continually upgrading their vehicles with constant reviews to the models, it goes without saying that all cars, even premium German brands, require regular servicing. Without ongoing maintenance, it’s hard to ensure that your Mercedes will perform at its best.

Mr K brought his Mercedes C220 into our Batley garage with an AdBlue warning light, and our Mercedes specialist technicians got straight to investigating and diagnosing the problem. RM Motors is an independent garage, taking care of our clients’ Mercedes, to ensure they get the most from their vehicles whilst receiving great service and value.

We pride ourselves on getting to the root cause of the problem by using our knowledge and expertise alongside our diagnostic evaluation process.

Our diagnostic evaluation makes sure that we can get to the root cause of the fault and, in turn, give the client a first-time fix every time. And as an independent Mercedes garage, our team uses recommended manufacturer guidelines, ensuring the integrity of your premium vehicle is maintained, just as Mercedes would expect it to be.

Mr K advised us that he had an Engine Management Light (EML) showing on his dashboard, highlighting a fault within the Engine Control Unit (ECU), as well as the AdBlue warning light.

Having topped up the AdBlue within the C220 already, Mr K advised that he had ended up overfilling the system. Unfortunately, what Mr K didn’t know was that overfilling your AdBlue system can further damage the complex diesel system. The sophisticated system relies heavily on having everything working within the correct levels.

Our team of Mercedes specialists took the C220 and got to work on completing our diagnostic evaluation, including a full visual inspection. Having looked over the engine from all angles, our experts couldn’t see anything immediately wrong, so we started the evaluation with a full scan of the vehicle’s serial data.


Within our garage, we accessed the vehicle serial data and found the fault code:

  • P140300 – The control unit AdBlue has a malfunction

Knowing the fault code and with our expertise, our Mercedes specialist technicians knew they had to access the AdBlue pump and start by checking the system’s wiring.  

With no power going into the AdBlue module, they began stripping back the wiring to locate the origin of the fault. The team found that the main loom had rubbed completely through, which had caused water contamination and some severe corrosion to the power wire.

The wiring had to be replaced entirely and re-insulated, a service our expert Mercedes technicians can complete at RM Motors.

Remembering what Mr K had told the team about overfilling the AdBlue system, our team checked and noticed a problem with the level sensor. Our knowledge and experience have taught us that if you overfill the AdBlue system, the ultrasonic sensor cannot check the level and therefore causes misinformation within the vehicle’s systems.

We needed to drain the AdBlue tank system and start again. After re-filling the tank to the correct level, we took the C220 for a road test and confirmed that the Engine Management Warning light had gone and the Mercedes was back to being a premium, reliable vehicle.


With the AdBlue system put back together and the road test completed, our team of Mercedes specialist technicians gave the C220 a final check over and handed the car back to Mr K.

RM Motors gave Mr K that first-time fix and returned his C220 to him within 24 hours, saving him the stress and money of repeated repairs. Everything was explained in detail to Mr K so that he was fully aware of what was going on through the repair, and therefore he knew he wouldn’t experience unnecessary costs.

This transparent approach is key to our business at RM Motors. With client safety a priority, we ensure our clients are fully involved in the process of maintaining their vehicles.

So if you are a Mercedes owner and want to enjoy the reliability of your vehicle, contact RM Motors today and let our team of Mercedes specialists look after your car, giving you peace of mind and great value for money.

Call RM Motors today for all your Batley Mercedes servicing and repairs needs.

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