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How did you check codes?
Have you tried disconnecting the battery to reset the ecu (computer) ?
Did you check the codes with the self test method?
What make, model, year and engine size are we talking about. Does the light actually say O/2 sensor or something else?
I think your just not reading them right.
I'm giving you this info for free because Jon_050 has already posted a paying answer here and you should accept his earlier for this.
The computer is usually located in front of the console.