Warning - Get drivetrain checked soon. Possible to continue journey
 

I drove halfway around Australia in my 2011 520D, I clocked up 6,900km on the journey and since then there have been issues with oil and water leaks so have replaced a lot of parts
Now I am getting a message popping up saying check Drivetrain when I turn off the car
I took it to BMW for a trade in and they connected it to their computer and the error is as follows:

Code 25DB00 - Exhaust back pressure sensor upstream or diesel particulate filter, signal: Short circuit to B+

I have checked everywhere but can't find any posts for this code. The sales guy at BMW said it may need a DFP clean or replacement which will costs $$$$

Has anyone had any experience with this issue?