Keeps the exhaust gas out of your engine. It's a small plate that goes between the up pipe (passenger side Splits off to divert some exhaust gas back into the engine via the EGR cooler then the intake) and keeps all that nasty exhaust gas out of your engine. Pretty simple install, but sort of hard to get at (back near the fire wall, but it's not that bad. Topside creeper is a big help.