1990 honda civic lx. it keeps throwing code 1 for O2 sensor.. i just replaced it with a brand new one from autozone and reset ecu... still throws code. but only over 80MPH on freeway. Any ideas why guys?

 

long shot, but take off your heat shield for your exhaust manifold. see if your manifold is cracked, right about the o2 hole.

 

A CEL code does not usually tell you what is wrong.

You'll understand this if you get the service manual and look up code 1. Code 1 tells you that there is a problem in the O2 sensor circuit or in the fuel system. If the problem lies in the O2 sensor circuit, then a bad O2 sensor is only one of the possibilities. Others include a bad O2 sensor wire (short or open) or even a bad ECU.