Physh1
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Nope...they are there to check for catalytic efficiency. The pre cat sensors regulate air/fuel mix & tuning. All 4 are important if you have cats and the pre cat ones do more work keeping air/fuel correct.The back two O2 sensors are the most important (they do more than check for cats - they measure how well the cats are working). I would replace those first, as they are the most likely to cause your O2 sensor issues.
Cameron