P0652 code what is the fix?

Question

Hi,

I have a mk6 golf gt tdi and recently I'm having problems. It's showing a fault code P0652 sensor reference volrage 'b' circuit/open. I have no idea how to fix this.. I was told by a garage ut was a faulty egr valve and I've spent a silly amount of money putting a brand new egr valve on. The car has been remapped previously but has been running fine for 2 years before this issue has started. Any ideas where I should start with the repair?