That would be the DPFE(differential pressure feedback) sensor.
The two hoses hook to exhaust system one closer to EGR valve, one farther away, as computer opens EGR valve the pressure in the closer hose will drop more than the farther away hose.
With this pressure difference between the 2 hoses the computer can "tell" if EGR is indeed opening, and how much it is opening so it can vary the amount of exhaust gases entering the intake.
Actually a pretty good flexible system, older EGR systems would clogged up so less and less exhaust gases would flow in, and no way to tell that was happening, so you would fail emissions test and start to get pinging.
And I forgot to mention in my first post about spark timing.
With an EGR system spark timing can be more aggressive, to get better performance, so computers are set up differently to take advantage of that.
This is another reason an engine will get pingy when EGR is removed.
And you can't adjust spark timing without a computer programmer.
Last edited by RonD; 08-14-2016 at 10:36 AM.