I think that I had the O2 (P0151) and Emissions (P0449) on my 2009 Chevy Avalanche, last year. Everything was fine and then one day, I couldn't get out of 3rd gear (red line & still topped out at 40 - 45 mph). For me, they didn't have to rebuild my transmission, but they did have to replace the emissions solenoid and did a complete transmission fluid flush (drain, flush & refill w/clean fluid). Being that it wasn't throwing codes before the rebuild, you might want to ask them to take another look to make sure that there are no fluid leaks, you have all your fluid, no vacuum leaks. But, I know for my truck, it took the shop's senior mechanic a couple days of digging to find out how to correct those code issues; I'm not sure if a Chevy Dealership's maintenance department would be better suited for that type of work or if they'd have the same issues. Here's what I found on those codes (my apologies if this is redundant information) --- U1000 - http://www.aboutautomobile.com/TSB/2003/Chevrolet/Avalanche/Transfer+Case (4WD / Transfer Case) | P0151 - http://www.aboutautomobile.com/DTC/P0151 (O2 Sensor) | P0300 - http://www.aboutautomobile.com/DTC/P0300 (Cylinder Misfire) | P0449 - http://www.aboutautomobile.com/DTC/P0449 (Emissions Solenoid)