Mercedes SCR / Adblue Fault

Mercedes AdBlue Problems – Background

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

This process transforms up to 90% of the harmful nitrogen oxides made by the vehicle into nitrogen and water. Modern diesel-powered cars merge 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 kept in an external tank, which has an internal delivery pump. From there, it is fed 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 from happening.

AdBlue and Activation is managed by 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.

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

Common faults

Although fairly simple, this system is very sensitive and delicate, some of the common faults include:

  • in-tank heater element, which burns out. The only way to solve it is by replacing it with a new one.
  • injector and connecting tubing clogged up. This system works only at high temperatures, urea solution can crystallise if you do short journeys.
  • NOx sensor can break or give false readings

Countdown warning system

If there is a issue 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 iasmportant to solve any issues regarding the AdBlue system without delay.

Different models will give you, 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

There are ways to reset Mercedes limited start condition, you need to find the root of the problem and fix an AdBlue system that is not working. The two most common problems are:

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

No matter the cause, an AdBlue system that doesn’t work luckily will not have any effect on the engine operation or performance. You can easily drive around for years without AdBlue, without affecting your engine. That only works sometimes though, b a 2010 and newer Mercedes BlueTec will  likely go into limp mode to prevent you from driving it.

2009 and older Mercedes-Benz BlueTec cars 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 different warnings if the AdBlue system is not working correctly? Well, the answer for this is a very simple, that is the only thing that can prevent you from driving your Mercedes like that.


Make sure first of all, check the AdBlue level and top it up if needed. You can buy AdBlue fluid from most Mercedes dealer, or you can get a Diesel Exhaust Fluid from auto part stores or petrol stations.

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

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