ok I have seen older posts related to this but here ie my senario. 02 sentra SE-R just replaced engine with a 06 altima 2.5L engine complete. I used intake fuel injectors and throttle body. got everything hooked up car started up fine after fuel and oil got to everything. drove car for about 5 miles. turned off car check for any leaks. nothing. drove car to fill up gas tank to chk mileage with diff eng installed and the SES lite came on. before eng install no problems with the sensors. had installed the #1 o2 sensor about 1,500 miles ago. time is about a month, because of the precat failure causing oil consumption thus the engine change. now with the new eng I did added a hotshot header and put the #1 o2 senser in the hole that is in the last header pipe cyl #4. the #2 o2 sensor I moved back behind the 2nd cat and extended the wiring harness on that sensor up to the harness where it orig plug in by the exhaust manifold. the wires on the #2 sensor were soldered.