Diesel Exhaust Issue

[carobin]

here's one that the experts can't seem to resolve....there is a persistent error (46C4:Reducing Agent Pressure Line Heating Activation) which I have seen some discussion here about but wanted to start a new post. My mechanic has replaced the line from the DEF tank (which has a heating element to it) and that didn't fix it. he also feels strongly that this is not related to the active tank. he is charging me to replace parts that do not fix the issue - is this standard? any suggestions on what else this could be? I want to sell this car. it has had all repairs done and this remaining check engine light will be an issue. car has 145k on it.