The sensors used in modern vehicles provide a LOT more information about the fuel mixture and are accurate under a much wider range of conditions; They're known as "wideband" sensors and they do serve as a diagnostic tool.
Older types of oxygen sensors (aka "Lambda" sensors) are only capable of detecting a very specific mixture; get outside of that range and it provides essentially zero data. They're useless except as a trim sensor to adjust for that 1% that the mixture may be off... Your mileage may suffer from a bad one, but it's not going to stop running.
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