Sorry for the length of this. OK my 350 Vortec has had both banks running rich for quite a while. I put 2 upstream O2 sensors in it over a year ago(my attempt to fix it then). When it went through emissions the mechanic doing it put a Fuel Pressure Regulator in it and the Rich COndition seemed to have gone away and it passed emissions. Now a year later, the condition has been back for a little while, it gets bad mileage(about 11) so it is in fact running rich. The code is mildly intermittent(light goes on and off so its passing plenty of drive cycles obviouslly). It went off long enough once that I got it through emissions this year, including the required sniffer test. Then the codes came back. Finally a guy at work gave me a spare Mass Air flow he had. It was replaced because the truck ran like crap when it was cold, but it never tripped a code. So I put it on figuring, long shot but why not try it. Sure enough, 3 weeks of using the MAF, it ran like crap cold but the Rich code was gone. So I confirmed on Monday there were no pending or stored codes and ordered a brand new Mass Air from a local Chevy dealer(AC Delco, no aftermarket crap). I installed it today, the truck isn't making the MAX power it has ever, according to the old seat of the pants dyno, but I didn't care as long as the codes were gone. Well, tonight they both came back(P0175, P0173, Too Rich on both banks). Now, this isn't asking how to fix the rich condition, but can someone explain why the hell the other Mass Air Flow didn't let the codes come up? Now if you want to take a crack at "fixing" the problem, be my guest.