Usually a bad 02 sensor will throw an error and cause the Check Engine light to appear. It will also show up in a vehicle running poorly when it first starts up or once it has warmed up and a drop in power or mileage. This is due to the 02 sensor giving out bad readings. I had an issue where it ran great when cold, poor when warm. I also had a Jeep YJ that would develop a miss when cruising once the engine warmed up due to the engine running lean.
I believe they are a 60,000 mile replacement item, so it might not be bad idea to replace it along with the standard new plugs and wires when performing a tune-up.
