I'd like to add that the engine will run just fine, probably more power and smoother if the O2 sensor is reading lean. More gasoline, more power. It's easy to check the O2 sensor. Attach a digital voltmeter positive to the black/dark green wire, negative to the engine block. The voltage should swing between 0.1 and 0.8 Volts DC continuously.If the sensor is new and the reading is stuck at 0.1 Volts you have a crack in the exhaust manifold, end of story.