I just finished putting the head back on my car and drove it around easy for a couple of days and it ran fine. No CEL's and ran great. Now I am getting a CEL and it is code P0401(I think). It says insufficient exhaust gas flow??? I check the EGR valve to make sure the line was good and that the valve was working and it works fine. I took the vaccume line off of the sensor and checked it for vaccume, its got it, and put that on the nipple of the valve to hold open the valve. It works because the car shut off, so the valve is opening and closing as it should. Please tell me what else to check? I am lost now, unless its a bad sensor.