this is a new issue and plugs have been replaced. The only sensor I can think would be in issue would be the MAF. It measures the ariflow if it was off the car would run rich. If the O2's were not responding right I am assuming the would have a code thrown. is there any other places the proablem could be coming from. no light is on but milage is down and the exhaust smells rich.
That should work out well for ya!