2000 grand am with SES light on codes as follows p0300 and p0420 ... Random missfires and Catalyst system efficiency below threshold (Bank 1) .... Both up and downstream o2 sensors have been replaced . new spark plugs wires and 1 of 2 coil packs replaced. When I first start it, it has a hard time getting idle right then its fine and drives like s dream. It does it hot or cold.