Have Subaru Questions? Ask a Subaru Mechanic and Get Answers ASAP
If you look at the engine you should see a round plastic disk looking item. This is the BPD valve. It regulates some of the vacuum to the egr. If you pull it off and tap it on your hand you might get some water out of the bottom hose. If you do, replace it. This is the most common cause of the 400 code on these.
Please let me know what more I can do to assist you today. Also, please remember to rate my answer, Just Answer does not compensate me for my time and resources. Follow up questions are included in the price of your question. If you are using the free trial, a rating is still required.Thank you so much!