I'd like to know what the rear oxygen sensors does?My guess measures to see if exhaust gases are clean,and runs the engine leaner.Since on my car I don't use cats,because of the NOS.I'm wondering can I simply solder a resistor in the oxygen sensor plug? Looking at this thinking I'm running lean.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

or could I simply plug hole,and expose sensor to air.
Did some research it seems I need not do anything,by all rights I should have thrown a code.This tells me even though a computer scan, revealed that oxygen sensors were working 100%,may I should replace primary sensor as it has 170,000 miles?

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1999 w140, quit voting to old, and to old to fight, a god damned veteran