1998 Chevy Lumina Egr code P0401

i have a '98 lumina with a 3.1 v6 . for the last 6 months the check engine light has been on . when scanned gives code p0401 - egr- insufficient flow. first thing i did was remove the valve and clean out the ports , code came back . i then replaced the egr valve and map sensor(even though i couldn't find anything wrong with them , just wanted to eliminate ), code came back with in a day of erasing it , i had the intake manifold completely off last month to do a gasket , recleaned the egr passage , clean out the tube running from the exhaust manifold to the egr . code still comes back . i'm convinced that the computer is either reading it wrong or another sensor is bad. i purchased an auto xray 6000 scan tool , which will be here tomorrow . What sensor readings , info , ect.. should i look at ? i've never used a scan tool before