The high voltage means that all available oxygen is used in the exhaust gas, and therefore the dis-similar oxygen content from the exhaust side to the ambient air side generates the voltage - hence the engine is running rich. To test the O2 sensor, add air to the intake system, without causing a misfire. Remove the power brake hose and put your thumb over it. With the engine running, and while monitoring the O2, add a bit of air by lifting your thumb a small amount. If the sensor quickly shows a lower voltage, preferably below .200 volts, then it is probably OK, and you have confirmed that the fuel system is otherwise at fault. If the O2 won't respond, or get much below .300 volts, then it is faulty. It's a quick check, but it works. You may also find the engine smooths out.