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(1) Look the Engine Bay over for any obvious Hose-Vacuum Leaks around the Vacuum Lines and Intake Manifold. Use a Can of Starter Fluid with the Engine at idle and spray small amounts around the Intake Manifold Flanges. If the Engine RPM suddenly rises... You'll know that the Intake Manifold Gaskets are leaking Un-Metered Air and only struggles briefly at idle on Cold Starts until the Motor Heats Up and seals them off a little better. Be Careful with the Starter Fluid... It will be easy to have an Engine Fire if this stuff suddenly flames over.
(2) A Very Dirty Mass Air Flow Sensor (Use CRC MAF Sensor Cleaner ONLY to Spray Out the Screen and innards and then apply a Fine Brush to VERY GENTLY but thoroughly clean the Delicate concealed Sensor Element-Resistors of any clinging Debris).
(3) If your Vehicle is a Flex/Fuel version... investigate how to do a Fuel/Alcohol Composition Re-Set. Please know that regardless of whether or not your Engine is throwing Lean or in this case RICH F/A Non-Stoichiometric Codes for both Banks, this doesn't change the Gasoline to Alcohol Ratio inside your Fuel Tank which should MAX out somewhere around 9% for Alcohol content on average from your local Gas Stations,
(4) Investigate finding a Non-Dealership Local Shop to gain 'less pricey' access to a High End Scanner that can Update your Truck PCM for adjusted Flex-Fuel and any New Calibrations and then seek out any TSBs that cover this issue, broad spectrum , for the 2012 Sierra-Silverado Trucks.
(5) Please advise in detail on how you have been using your Diablo 2 Performance Tuner on this vehicle, as anything Non-Standard will get wiped out if you have those OEM Calibrations Re-Sets and Updates performed. Ultimately, you might have to bring things back to a "Stock-as-a-Clock" Baseline in order to sort these problems out if they prove to be "Tune" Related.