O2 will usually throw a code if its bad. it also has very little to do with mileage. Check the TPS first. Also check the coolant temp sensor(not the one for the gauge. There is a different one for the computer)and the cold start injector circuit(first make sure the injector isn't leaking, then check the time switch which is right next to the engine temp sensor). Have you done a tuneup lately? Change your plugs and adjust the valves. Also make sure all the vacuum lines are intacted.
