That's there for the California Emissions package, which uses four O2
sensors (it adds a pre-cat and post-cat to the existing two sensors in each
Why would you want to do that? First, there are no further "upstream"
sensors than the ones you have now - one for each bank of cylinders.
Second, you'd need to re-flash your PCM with the CA Emissions program, which
will cost you some power.
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