is it the o2 before or after the Cat? If it's the one before, it's a good idea to go ahead and replace for that o2 sensor lets you know if your running lean or rich. If it's the 2nd one... that simply tells you if your cat is doing it's job.

How to bypass? I simply typed into yahoo + search + "o2 sensor bypass" = http://www.vfaq.com/mods/O2bypass.html