2006 Chevy Uplander 3.5 auto...I just fixed dtc p0449...i drove it for about 15 miles and check engine light came back on...now it gives me code p0404....Please Help....I took off the egr..and cleaned it...COuld that be it? also researched it some and it said could be a clogged catalytic converter?? Please help...any and all advice would greatly be appreciated.