EGR= Exhaust gas re-circulation

Basically just vents some of the exhaust back into the engine to "re-burn" it to lower emissions. I remove it from all my vehicles that have it. Local emission laws may not allow that, but it can still be disabled. In my experience it works fine when new, but as stuff ages, it causes nothing but problems. Especially on a diesel with all the soot as opposed to gas engines. Gas or diesel, it will still get carbon build-up. One day it will stick, hopefully closed, but maybe open, and then you have an engine that will idle poorly if at all.