your two o2 sensors are placed in front and behind the catalytic converter. The first one monitors how much hc's, co's, and o2's are coming directly out of the engine. The second one monitors the catalytic converters performance by reading for the same thing the first one is reading. With that information sent up to the ecu, check engine lights will usually come on. Because of the codes that came up, it is more than likely your o2 sensors. Your first o2 sensors detect what the fuel mixture is in the engine,by sensing o2. It then sends that information to the ECU, who changes the fuel mixture by grounding the injectors open longer or shorter. That is probably why you feel a difference after 55mph. At highway speeds, you car needs less fuel to run because of the increase in air flow. So more than likely your running a little richer than before. hth