1996 Mercury Sable AUTO ZONE DIGNOSTIC EGR

Engine Mechanical problem
1996 Mercury Sable 161000 miles

I went to auto zone for there free dig. It pulled up EGR. I called several places where I could get parts and labor. One person said that a good dign, was not done. That it could be a back pressure sensor. So I'm not sure to pay the money for a full dign on it, and chance it being just either the egr valve or solinoid or just go ahead and pay for it. We had to put a new speed sensor on it during the summer. Ran fine, tell the check engine light came on & also the I.P BREAK LIGHT came on. It didn't mess up tell I was in a line where I was on the break for a few min and it wanted to kick. Did fine the rest of the way home. Can anyone help.