2002 Ford Taurus Fuel Mix Too Rich + Sluggish start

Recently my Taurus would not start. After awhile, we were able to get it started and taken to the mechanics. He replaced one of the rear oxygen sensors and the fuel filter. He also put in some fuel injector cleaner. However, we are still getting an intermittent sluggish start and a check engine light indicating that the fuel mix is too rich. We've taken it back to the mechanic and he is scratching his head over what it could be. As near as we can tell, the Check Engine code is still pointing to the same O2 Sensor.

We're almost at a point of giving up, but were hoping to find some additional advice on how to continue troubleshooting this problem.

Thanks for any info you might be able to provide.