why is my service suspension light on and i keep getting a code po121 i replaced my gas cap canister gas pedal and my auto start want work but when the code is gone it works if i take the

On my 2009 Avalanche, there was an emissions sensor that went bad. I tried all the things you did, which worked for a while then the check engine light came back on. Before I took it to the garage, I lost power at the transmission - pedal all the way in, RPMs to the red and 40 mph, on a major highway. ... it was ~$800 to fix the sensor & flush transmission fluid. Runs like a champ now. Might be worth having your mechanic take a look. Hopefully it's not as expensive a repair as I had.