rear o2 sensor is giving me code p0140 which reads as low/no activity. which is not good. since it is 1 yr old. folks say no big deal, it just monitors cat output and has no affect on fuel mixture. except i get a daily cel light which is annoying. clear code, 10 min later code comes back. not sure how often computer actually checks sensor performance? constantly? if i drive on hwy at steady speed, the cel stays off quite awhile. but any stop/start at stoplights seems to trip a quick light. what does low/no activity mean? does that point to a dead or failing sensor?