I've been on this forum b4 and the discussion has come up about carbon deposits giving a P401 code alarm; where exactly should I look/clean these carbon buildups? I've replaced the EGR valve and DPFE sensor (used an Ecklin dpfe sensor but am replacing it with a motorcraft sensor) but still have the P401 code; thought maybe the solution could be these carbon deposits but am not sure where I should be looking; can someone give advice about this? Appreciate your help, thanks.