2003 Chevy Avalanche engine running rich .

P0300 is random multiple misfire, as you haven't put the mileage on the questionaire, try replacing plugs and maybe wires if they are bad as well, check those with an ohm meter (5000 per foot) make sure to wiggle wireswhile checking. If the codes return after clearing , check or a vacuum lines with holes or disconnected. then you may have to have injectors cleaned professionally. Also replace air/fuel fiters as well. P0172 &175 says both banks running rich. that may cure your problem.