Found a disconected plug? what is it?

[Dublinoh]

Had no idea how to search this, hope someone can tell if I should connect it. 1987 Carrera drivers side engine compartment near the fuel filter:

[88911coupe]

pretty sure that's the O2 sensor but I'm sure one of the resident experts will tell you for sure.

[CorsePerVita]

That looks like an O2 sensor to me as well. Where does the rest of it run to? If it runs down past the engine near the exhaust then it's your O2 sensor.

[Dublinoh]

Yes it does go down to what I believe is the top of the oxygen sensor. Is there any reason someone would leave it disconected? How would that affect engine performance? Thanks

[CorsePerVita]

Depends on why it was disconnected. If it is your O2 sensor, if the car had issues running it may have been unplugged to mask other issues so it runs in open loop. Mine was unplugged when I got my car (although I've got a different engine than you and mine is CIS) - the car ran horribly rich and had crap mileage. Reconnecting it made the car run lean but no longer in open loop. Turns out the car had vacuum leaks out the ears (which have since been resolved). So I assume they unplugged it on mine to mask the slight lean condition it had. It acted very strange indeed with the idle issues the car had till I got it all sealed up.