Diesel Exhaust Fluid Heater (DEF Fluid) is a known weak points. All Bluetec vehicles use Diesel exhaust fluid, also known as AdBlue. It is stored in an external tank, and it would freeze in cold weather. To prevent this, DEF tank is equipped with an electric heater which can burn out.
Common problems of DEF Fluid: 1. Diesel Particulate Filter. As Mercedes DPF traps and holds soot, with time it reaches a point when it needs to be ?emptied out?. 2. DPF Exhaust Pressure Sensor. Mercedes, like many diesel cars, uses a DPF Exhaust Pressure Sensor to determine how much does the DPF restrict the exhaust flow. Diesel Exhaust Fluid Heater.
The Diesel Exhaust Fluid Heater, commonly called the "DEF Heater", is located in the DEF (Diesel Exhaust Fluid / Urea) tank and actually contains the pickup tube for the reductant pump module, an electric heating element, a temperature sensor, a pressure sensor, and an electrode-type level sensor. It enables the SCR (Selective Catalytic Reduction) to function at low temperatures.