replacing o2 sensor

My daughter has a 97 monte carlo z34...it will start for about a few seconds and then go dead with the lights going dim everything..my husband thinks it is the o2 sensor. the computer said that it needed changed, it that what it does when it needs changed?....and how much do I need to expect to pay a garage to change it?

Neither one of these thing are right, check and tighten battery cables first, then you will need professional help from there on.

Yeah, that problem sounds electrical, like a loose ground or bare/burnt wires.