Hi everyone, I have a 91 honda civic si that I have replaced the o2 sensor on and the check engine light still won't go off. Can any one help?

You need a scan tool to verify it was a faulty 02 or something else. You will also have to reset the ECM. It won't do it by itself.

Disconnecting the negative battery cable for 10 seconds will reset the ECM.

Agreed disconnect the battery cable and it should go off. If it comes back on after reset there is another problem somewhere. Good luck!