I have a 2000 civic ex. It drives perfectly fine other than a high idle (2000 then down to 1500 then to 1100rpms) until the engine warms up. Once the engine warms up the idle returns to about 1500 rpms and it hesitates when i try to accelerate. It also "jumps back" when i let off the gas. I have gotten the codes read at autozone and it came back with a misfire in 1 of the cylinders, bad o2 sensor, and iat or iac sensor (cant remember which it was). the o2 sensor on the manifold is there but is not plugged in and dont see where it should be plugged into here's a pic of the plug



and one closer

 

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that o2 plug should be around there somewhere get ur hands dirty and look around those wires.. i dont think there is somebody dumb enough to chop it off..

 

^^^^hes right. jack it up and look underneath everywhere too, my boys got tangled and it took awhile to find it