Read up on technical info on oxygen sensors and how they work. Bosch website has good info. The heated ones can become sluggish when dirty, but otherwise they either work properly or don't work at all. The sensor is part of a feedback control system that regulates the mixture. Like other feedback systems they keep pretty close tolerances (in this case mandated by federal government). When the oxygen sensor fails completely and produces no signal at all or an extreme value unrelated to oxygen, then the gas mileage and emissions will go very bad and you will get your 'check engine' light. It makes no sense to prematurely replace one.