Dealer said O2 sensor, $750 plus labor. We found a broken fitting on the valve cover where the vent hose connects, $250+ for a new cover, or my neighbor's black silicone material (no motion, so glue fixed it) but the code came back, so maybe it's the O2 sensor after all? Back to Allen Tire for a more detailed analysis (my $39 Costco code reader is pretty basic!). O2 sensors on line range from $39 to $289, depending on which one, and who makes it. Dealer's $750 is ludicrous.