Welcome onboard.
The oxygen sensor you mention is the most difficult to access of the four on the v6. Not impossible, but not the one to start DIY training.
The pre-cats are between the 2 sensors on each exhaust manifold. The upper sensors input to engine operation (important), the lower ones check performance of pre-cats (less important).
98 Mystique LS v6 atx 110,000km
97 Contour LX v6 atx 125,000km
stock CD on LS, cass on LX
spoiler, tinted windows on LS only
4 wheel discs on LX only
All other available options on both
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