Peugeot 206 Ventilation Engine Cleaning

Hello my friend, I have become a new member of your site. I have read a few articles from the "how to" section. You look very experienced in 206. What can I do to make the 2010 206+ 1.4 HDI lower than the oxygen sensor's perceived oxygen level? I tried via ECU but I couldn't intervene. I would appreciate if you know or direct someone who can help, or if you have knowledge about the subject. thanks in advance