I have a P20B9 code. At first, based on threads and recommendation of a local shop, I changed the def tank heater and nozzle (xemodex) and the larger supply line (pressured) but I still get the code. Any insights what else it could be?

prefer not to do a “delete”

fault codes mostly don't go away on their own - the dash EML light often goes away

but the list of mostly fake junk a W221 collects every day is just that - clear it all out drive for 10 mins any real faults will be straight back

Seems there is a distinction between a stored vs current code. Current means the fault is present. Stored means it was once present (and could be quite old, depending on the code) but not currently and is therefore much less meaningful.

its a current Current Code.
when Def Heater was replaced, codes were cleared, check engine light came on after 20 mins & Count down reinitiated after ~8 starts
replaced Urea A line (Long) and Nozzle Replaced, Code as listed above came back after car restarted after initial clearing; count down has not started and 16 starts since reset.

lots of issues with adblu on Mercedes cars - there were many that had a faulty sensor, failure to do as instructed often leads to tank, pump and sensor replacement as the only fix that works (at crazy money)

seems to be a new manu rip off weapon of choice - just did this to a mate with Peugeot - look on the forums its normal on this brand too

which sensor was replaced?