I bought a used 2001 Pathfinder from my Nissan dealership back in May with 36,000 miles on it. I now have 42,000 miles. A couple months ago the Service Engine Soon light came on so I brought it in to get checked out and they said my Intake Sensor was bad. Luckily it was covered under the limited warranty since it happened within 3 months so it was a free repair. The other day the light came on again. I brought it back to the dealership and after charging me $82 to diagnose the problem, they said my O2 sensor needed to be replaced. After telling me it would be $250 in labor and $70 for the part, I felt this was a bit much in labor costs so I bought the part from them and went on my way. When I got back in the car, the engine light was off and it has not come back on since. Any idea why the light wouldn't come back on? They mentioned something about "reprogramming" the computer that monitors the sensors as part of the labor. Does that mean until it gets reprogrammed I will never know if a sensor is bad or was there possibly nothing wrong in the first place and by "diagnosing" they reset the sensor monitor and that is why the light isn't back on? And my second question is, how important is it to get an O2 sensor fixed? Is it an urgent repair or can it wait? How does it effect engine performance, etc...?