My dad had the "check engine light" come on inside his 00' Maxima after he bought it. The mechanics said it was an o2 sensor, but they didn't know which one...and wanted to replace all of them at about $650 total. He told them he would just straight pipe his car (out of anger) and never fixed the sensor.

First off, how many sensors are there (I have seen one...by the rear exhaust manifold unerneath)? And how do I go about troubleshooting which one it is? The sensors themselves are very cheap anyhow, cheaper than my non-in-demand 3rd gen o2 sensor at least.

It happened right after he bought it...he said he blew the cobwebs out on it and the "engine" light came on; he said he then noticed that there was a jangly sound coming from the exhaust and it turned out to be a loose flange. He had the flange repaired. I'm only adding this part in because he bought it for $11k and it was worth $16k and all of that **** happened right after he bought it...(makes me wonder if this was a reacurring problem and the previous owner ditched the car knowing this) so is there another problem that caused the o2 sensor to fail and the flange to come apart? Or was he just driving too fast/long at high rpms? lol