O2 Sensor Replacement Symptoms

[morontron]

Hey what are some of the symptoms that indicate I need a new O2 sensor?
I was talking to my auto instructor and he was telling me that if I needed a new one my check engine light would come on but it seems to me like that would only be a really drastic case...

[KenC]

Usually the car will run rich.

[DarkWing6]

When mine went out my check engine light would come on from time to time and my car would idle funny. It would occasionally idle high, sometimes low, sometimes perfect. Also, as Ken said above, you will run rich.

[arsevader]

Hey what are some of the symptoms that indicate I need a new O2 sensor?
I was talking to my auto instructor and he was telling me that if I needed a new one my check engine light would come on but it seems to me like that would only be a really drastic case...

^ These guys are right.

Usually the first thing to happen is that the O2 sensor gets sluggish which probably wouldn't throw a CEL but will cause drivability issues.