Solved my case after 4 years of not figuring out why p1406 wouldnt go away and couldnt passing smog the distributor gasket would retard and leak spark out to the e.g.r valve which is right next to it and short out the e.g.r valve over and over I went through 3 e.g.r valves and till I figured it out the gasket for some reason under heat and oil conditions stretches and the problem persists over and over again I so this time I sealed new distributor gasket with black rvt and put a new delphi egr valve on and no problem what a headache for 4 years I took 2 to shops paid $400 each time nobody could figure it out i cleaned every port and still no good to make a long story short I got shocked while I was working around the wires so I open the hood at night and I found the location and everything is fine now p1406 code could also be a false code for this problem for a short on EGR valve from distributor Leak