Please excuse my ignorance, what exactly do O2 sensors do. When they go out what problems do they cause? I ask because I think my rear O2 sensor is out and the car is running perfect. Is this something I should worry about replacing? Or just get my SES light reset.
Thanks for your help!

 

The emissions test will fail for sure. Simple reset won't work.

 

There are four O2 sensors on your car, two before the cat and two after, so make sure your targeting the right one(s). Their purpose is to monitor the exhaust gasses and cause the adjustment of the engine's timing, firing, etc. (based upon the readings) to maintain optimum performance, efficiency, and clean emissions.

I just had this exact probem resolved on my 2000 at the stealership. The front two sensors needed replacing and they were happy to do it for a mere $560.

If you're sure a sensor is the cause, you can replace them yourself. I found our O2 sensors on www.oxygensensors.com ranging from $60 to $100, depending on if you want OEM or not.

I'm not sure how you'd have the SES light cleared if you were to do the replacement yourself, though.

 

After any repair like that the CEL must be reset... the instructions are in the 4th gen forum. All you need is a flathead screwdriver and 5 min.