Why replace all? Generally the pre-cat ones get lazy and give off an "aging" code because they have been in the dirty exhaust stream so long. So maybe you replace the 2 pre-cats. I drove for 2 years just having replaced one then replaced the other just because I felt like it. Didn't really make any difference.
oxygensensors - mikefocke2 will give you how to do it or get it done tips as well as part numbers
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