Should an O2 sensor upstream be changed before downstream?

There is no maintenance interval since these are not routinely changed. if you have a bad one or one that's become lazy (slow switching) or very high miles or mil on with code you might change them. If you have a bad one upstream, you should do both, since other is usually right behind it. These are more important to the actual running of the engine since injector timing is based on what they read. The downstream or one behind the convertor I wouldn't worry about too much unless you actually get a code for it-all it does it monitor catylist (convertor function) Convertors never wear out-if your engine is running right. People only replace convertors if they get "killed" by a poorly running engine-raw fuel superheats the catylist and they fry. Important note-upstream and downstream sensors ARE different, although both upstreams will probably be the same part no. Buy the right one and the job is easier with the special O2 socket.