Mercedes-Benz bulletin LI49-20-N-080376

The check engine light is illuminated, or the AdBlue warning is active, with recorded faults related to AdBlue quality or concentration. Complaint Customer reports an illuminated check engine light and or an AdBlue system warning indicating AdBlue quality out of specification. The vehicle may enter reduced performance mode. One or more of the following fault codes related to the AdBlue tank may be stored: P206B31: Malfunction of the concentration sensor for AdBlue; the signal is absent. P203B31: Malfunction of the AdBlue fill level sensor; the signal is absent. P300494: The quality of the AdBlue is insufficient. Background AdBlue must meet ISO 22241 quality requirements. ISO 22241 urea content limits for AdBlue are 31.8 percent to 33.2 percent by mass. AdBlue begins to crystallize and freeze at approximately 12 degrees F. Freeze-thaw does not degrade the product when uncontaminated, and the bulk concentration returns to normal after complete thaw and mixing. Cause In cold soak conditions and during freeze thaw exposure, the tank contents may not be uniform at the time of sampling or system checks. During freezing and partial thaw, ice formation can temporarily create concentration gradients in the tank. As a result, urea percent readings can vary based on sample location and the state of thaw and mixing, which can produce apparent out of range readings even though the bulk fluid returns to normal after mixing. Remedy Testing Do not use a single urea percent reading taken from frozen, partially frozen, or not fully mixed AdBlue to make a repair or responsibility decision. Confirm full thaw and ensure tank contents are mixed before sampling. 1. Confirm full thaw - Confirm the AdBlue tank and associated components are fully thawed before sampling. - Do not sample slushy or partially frozen fluid. 2. Mix tank contents - Perform a road drive intended to mix the tank contents before sampling. - Idle time alone is not a reliable substitute for mixing. 3. Sample and measure - Take the sample and measure with the MB refractometer per the MB tool instructions. - Record the urea percent value. 4. Interpret the result against ISO 22241 limits - Within specification is 31.8 percent to 33.2 percent urea by mass. 5. Repeat if out of specification - If out of specification, repeat after an additional mixing drive to reduce sampling variation. - If still out of spe

Owners of the 10 model years this bulletin names have filed 547 complaints with NHTSA since 2015. 9 of them name Fuel/Propulsion System — the same broad component group this bulletin is filed under. NHTSA groups complaints coarsely, so that means the same area of the vehicle, not necessarily the same fault.