nissanpark

The light of "service engine soon" turned on when I droved yesterday. My car was nissan sentra 2000 GXE, 69500 mile. I took it to the dealer to check it. The dealer said the o2 sensors had some problem. But there are 4 sensors, they didn't tell me how many sensors broke down, they told me it needs to took it out to find which sensor is wrong. I looked through some forum, usually they can tell me which sensor is wrong. The dealer told me each sensor costs $175,If four broken, it will cost 1000. I wonder it is the dealer's trick. so I didn’t change it yesterday. I found 2000 nissan sentra have some problem of foam coated the sensor to be recalled on 2003. but I didn't know this recall. Is anyone known if my car still under emission guarantee? Can anyone introduce some experience of change the sensors by yourself?