Home > Troubleshooting > Mercedes AdBlue Problems

Mercedes AdBlue Problems

AdBlue is just another name for Diesel Exhaust Fluid or DEF. This solution of urea in water is injected into exhaust gases as a part of the exhaust treatment system called selective catalytic reduction or SCR.

This process converts up to 90% of the vehicle’s harmful nitrogen oxides into nitrogen and water. Modern diesel-powered cars combine this with other emission control systems to meet the highest emission standards while retaining high engine power outputs and fuel economy.

How does it work?

AdBlue is stored in an external tank with an internal delivery pump. From there, it is injected into the stream of exhaust gases through the injection valve. As AdBlue would freeze in low outside temperatures, the heater element is installed into the tank to prevent this.

An Electronic Diesel Control unit or EDC controls activation and AdBlue dosing. This regulation is based on several parameters:

  • exhaust temperature,
  • NOx content before and after SCR catalytic converter
  • the operational readiness of the AdBlue system itself.

Be aware that the AdBlue system in your Mercedes will start working only when all the parameters are fulfilled.

Common faults

Although relatively simple, this system is very sensitive and delicate. Some of the common faults include:

  • In-tank heater element, which burns out. The only solution is to replace it with a new one.
  • The injector and connecting tubing clogged up. As this system works only at high exhaust temperatures, urea solution can crystallize if you have frequent short journeys.
  • The NOx sensor can break or give false readings

Countdown warning system

If there is a problem with the AdBlue system, your Mercedes will be limited to the number of starts or miles driven before it is not possible to restart it.

Dealing with any issues regarding the AdBlue system without delay is very important.

Depending on the model, you will get some of the following messages:

  • AdBlue Check Additive See Operator’s Manual
  • AdBlue Range
  • Engine Start Not Permitted (AdBlue Empty)
  • Check AdBlue. No engine Start is Possible in XXX miles/km.
  • Check AdBlue Remaining starts X.

While there are ways to reset Mercedes limited start condition (eBay link), finding the root of the problem and fixing an AdBlue system that is not working is important. The two most common AdBlue problems are:

  1. DEF fluid low level
  2. Fault with the SCR system.

No matter the cause, an AdBlue system that doesn’t work will not affect the engine’s operation or performance. You can easily drive for years without AdBlue, without affecting your engine. That only works on theory because a 2010 and newer Mercedes BlueTec will likely go into limp mode to prevent you from driving.

2009 and older Mercedes-Benz BlueTec vehicles and Sprinter vans can go over 300,000 miles on the original engine. These cars didn’t have the AdBlue system.

So, why do you get various warnings if the AdBlue system is not working? The answer to this is very simple – that is the only thing preventing you from driving your Mercedes like that. If there were a warning system like this countdown, most car owners would not top DEF when it runs out. And this would increase emissions and pollution.


First, check the AdBlue level and top it up if needed. You can buy AdBlue fluid from any Mercedes dealer or a generic Diesel Exhaust Fluid from auto part stores or gas stations.

If there is a fault within the system, the following should be checked:

  • Values from temperature sensor upstream of SCR catalytic converter
  • Values from NOx sensor control unit upstream of SCR catalytic converter
  • Values from NOx sensor control unit downstream of SCR catalytic converter
  • AdBlue tank for DEF quality and quantity
  • AdBlue delivery pump and injection valve
  • AdBlue metering amount
  • Visually check for leaks

How do I refill AdBlue?

This is as easy as filling up with fuel. DEF tank filler is located next to the fuel fill cap.  Just open the fuel lid and locate the blue colored cap. Remove the cap, and fill up the tank with Diesel Exhaust Fluid.

Make sure to add AdBlue to the correct tank. Do not mix AdBlue with diesel fluid. AdBlue is not an additive or a lubricant. Do not mix water with AdBlue.

All rights reserved Notice: CLOUD