will changing muffler activate o2 sensor error and turn on serv eng light on 2000 Nissan Maxima

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Getting service engine light activated by o2 sensor after muffler and exhaust pipe was changed
(1) Answer
it shouldn't, unless using modified part it might. bigger exhaust allows different air flow than stock model. sensor programmed for stock model. are the cats. still original. possible they may be clogging a bit. try clean fuel system . ethanol treatment to 1 tank of premium fuel. build up in fuel system, combustion chambers and exhaust can trip your sensors also.
may want to get diagnostic run to find any issues u should be aware of.