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Hi and thank you for your question. The way you laid out the symptoms, it sounds like all was fine for awhile then out of the blue the O2 sensor started throwing a code This is while no one had performed any service or repair the on bike. Where I am going with that question is it sounds like you had an O2 sensor on the bike the whole time, is that correct?
Have you removed the spark plugs to inspect them for color? Also, have you used any fuel treatments or stabilizers or any kind of through the gas tank chemicals?
Thank you for the additional information. At this point, let's pull the spark plugs out and see what they look like. We need some more info here. It sounds like the O2 sensor may have failed, providing it is still connected, but we want to be sure.