Items #1 & 2 are the pre-cat sensors that help to control car in Closed-Loop mode - hence the word 'regulating' in the description. They read the O2 level in exhaust right after combustion & send this info to PCM. If there is a problem with how the car runs, & it's related to the O2 sensors, then these are the suspect ones.
The other (2), items 4 & 5 are for monitoring effectiveness of cats - if the cats are going bad, these will tell the car based on after-cat O2 levels.
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