[MattB]

I just picked up an 02 sensor off the forum for a few bucks but it is a 3-wire (for 99's I was told) and mine is a 2001 (4-wire)

I want to know what the fourth wire does and if this 02 Sensor will work in my car or not... Here is a picture of what they look like.

 

[MattB]

well the plug is the same and i have been driving with it all day without any codes so... I think it is just a "signal" ground wire which was not used on the 3-wire sensors. Seems to be working fine though. Any input appreciated still!