I stated getting a Catalysis Efficiency code on Bank 2 earlier this year. So knowing it was going to be costly, I decided to wait until a better time financially. Well, last week I dropped it off at my mechanic and I opted to replaced both Cats since he had the exhaust system dropped, as well as installing a FlowMaster muffler & standard issue chrome tip. My shop drove it for about 30 miles to make sure all was good, and all was good, until I picked it up and drove 3 miles to my house where the CEL came on. I quickly pulled my scanner out and pulled about 5 codes ranging from rich condition to misfire all on Bank 1. Brought it back and my mechanic monitored the o2 sensors (with his very expensive looking scan tool) and found Bank 1 was not responding (switching from high to low voltage) as the others were and as it should have been. So he replaced the only that O2 sensor. Picked the truck up today and drove it about 35 miles to a friends house, stayed for about 1 hours then left. Got about half a mile down the road when the dreaded CEL came on again. Pulled the above code P0174 System Too Lean (Bank 2) & 2 - P0101 Mass or Volume Air Flow Circuit Range/Performance. Note on the P0101 - this code has pretty much come up with every other code in the last 6 months, so I'm not surprised at all to see it. It also has not trigger the CEL on it's own, so I'm not taking much consideration into it. I guess my next step it is to take it back BUT before I do so I want to do some basic checks myself. I'm going to start with spraying some carb cleaner around bank 2 intake manifold. I did replace the intake gaskets last year when I replaced both Knock Sensors, so it's possible their may be a problem there. Any other suggestions?