Nissan Repair: o2 sensor on 02 nissan frontier 4x4, nissan frontier, o2 sensors

Question
I was wondering how many o2 sensors this model has, and how long a replaced sensor should last? I had the service engine light come on april 2007(appx 50k miles) took it in to the dealer and had the sensor changed out. Yesterday, again, service light comes on.  My truck was just inspected two weeks ago and passed all emission tests. Should i take it back to the same shop, a new shop, not worry at all? What is your professional opinion? I really don't know what this sensor does - and don't want to delay repair if it means damaging my truck, but can't afford dealer repair costs every year for the same item...thanks, val

Answer
Valerie,

If yours is a V6 there is one oxygen sensor per side and one behind the converter for a total of three.  If you had one replaced at 50K miles then you ought to get the others replaced.  If all were replaced then you need to discover why they have failed again.  I would not think this would be the case.  So, go to Autozone or Checker and get the codes read for free.  There are specific codes for each and they should let you know which one is going out.  A defective oxygen sensor will cause reduced gas mileage since it makes the engine run rich.