My understanding is that the O2 sensors wear out with age and start to react slower and slower to changes. They won't necessarily throw a code till they completely fail. This past summer the way I found out mine were bad was because I failed emissions, after replacing my O2 sensors my CO and Nox dropped by over 50% and mileage went up. Never threw a code but they weren't reacting fast enough. There is a way to test but you'll need two people as they need to be tested with RPM's at about 2500 using a multi-meter.