Mitsubishi Repair: 97 Eclipse RS 2.0, O2 Sensor, gas mileage, cat converter

Question
Hello!
I drive a 97 Eclipse RS. It's a decent car, to say the least. I love it, but lately, the "Check Engine" light has been coming on. It doesn't stay on all the time, however. If I drive for a certain amount of time, it comes on. Once the car has stopped and been turned off for a while, the light stays off. I pulled the codes from the computer, and got code: 12, 21, and 55. I know what 12 and 55 mean, and I've researched code 21, the code for the upstream O2 sensor. What, exactly, are normal symptoms of a failing O2 sensor? I would hate to replace it, then still have the engine light on. Any advice would be of tremendous help. Thanks.

-SD

Answer
SD,
One of the most noticable signs of a bad O2 sensor is a drop in gas mileage.  If you noticed you're getting several miles per gallon less than normal, it most likely is the O2 sensor.  It's possible the catalytic converter may be clogged or failing as well, which can the O2 sensor code to be displayed.  If your exhaust smells like unburnt fuel and/or has a black color to it when you accelerate, that can also indicate a faulty O2 sensor as well.

You may want to have a mechanic inspect the O2 sensor and even the cat. converter if it'll give you more confidence in the diagnosis.
Good luck!