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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 harmful nitrogen oxides produced by the vehicle 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, which has 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.

Activation and AdBlue dosing is controlled by Electronic Diesel Control unit or EDC. 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 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 replacing it with a new one.
  • 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.
  • NOx sensor can break or give false readings

Countdown warning system

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

It is very important to deal with any issues regarding the AdBlue system without delay.

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 Possible in XXX miles/km.
  • Check AdBlue Remaining starts: X

While there are ways to reset Mercedes limited start condition (eBay link), it is important to find the root of the problem and fix an AdBlue system that is not working. 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 have any effect on the engine operation or performance. You can easily drive for years without AdBlue, without affecting your engine. That only works on theory though, because a 2010 and newer Mercedes BlueTec will more 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? Well, the answer for this is a very simple one – that is the only thing that will prevent you from driving your Mercedes like that. If there was a warning system such as this countdown, most car owners would not top up DEF when it runs out. And this would increase emissions and pollution.


First of all, check the AdBlue level and top it up if needed. You can buy AdBlue fluid from any Mercedes dealer, or you can get 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 of your choice.

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.

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