Fuel filter, spark plugs, plug wires, pcv valve, dirty injectors, dirty MAF, dirty air filter, etc. did you get the code pulled when the CEL was on? Usually if an o2 sensor is going out, it'll pop a code, you can reset it and it won't come back for a while until it completely goes out. But the o2 sensor usually goes bad because of something else, like one of the things listed.
