How do you replace a 2004 GMC Envoy oxygen sensor 1?

Sensor number 1 is the one before the catalytic converter. First off, are you sure that is your problem? There are numerous things that can give you a code for the oxygen sensor. A vacuum leak, a wire burnt on the manifold, and even a bad connection. Be sure before you spend money that you do not have to . It takes a special socket to replace it and the socket can not be used for anything else.