Generic code P20E8 refers to low pressure in the bluetec tank. MB code adds a 21 number to this code. Does this designate a subset to the low pressure such as a bad pump, fault wire connection, fuse, etc. .

 

No, it just means that the cheap OBD scanner is not designed like factory to further interrogate the control module and the system to further define the problem

 

P20E821 / P20E823 each code happened once in last week. My ICarsoft EU Pro (Gen 2, MBv01.35) tells as much as my $25 scanner does -- which is AdBlue Pressure Low, thats it. No further information whether pump or connection is bad.

AdBlue Tank capacity is 31 Liters (full tank). When error code generated, I have 21 liters in tank. So Adblue is not empty.

still looking for cause of error code
P20E821 / P20E823

 

If you are asking about a GL350, you are on the wrong forum.
https://mbworld.org/forums/gl-class-x166-173/