98 S70--Check engine light is on. Autozone checked their codes and said it was an O2 Sensor. Talked to a mechanic, who said that it may be I need new plugs because the carbon from the old plugs could be setting off the O2 sensor.Trying to make sure that my car will pass the emissions test. Alsocan someone tell me where the O2 sensor is, and if it is difficult to change.

 

Not exactly sure where they are on the S70, but they screw into the exhast system. Follow your exhast pipes and you will find them. You have one before the catalytic converter and one after. They look like the below image.