The O2 sensors generate 0.0-1.1 volts normally. Around 0.6 volts would indicate normal mixture control. Higher than that would indicate a rich mixture, lower would indicate lean. If you are getting a rich indication, you could have a miss, (bad spark plug, coil), or perhaps a sticky injector.
Could be a control issue as well, but I'd start with making sure all connections and grounds are good, and go from there. Is it throwing any service codes?
Hard to diagnose from afar. First, check all connections. Second, O2 sensor has to be hot, at least 600F to read properly. Most, with 3 or 4 wires have a ceramic heater to warm them up. As mentioned high voltage indicates rich condition.
If you have a code reader, run the car till hot, clear codes and drive again. If code returns you will need a diagnostic scan to pinpoint issue/s. Good luck.
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