The ECU needs to be reset to make the SES light go off if he fixed what was wrong with it. Unplugging the battery will not work on that car you will have to get a code reader or something to reset it. My moms SES light was on in her 2000 maxima because of the O2 sensor and the only way we could get the ECU to reset was to get a code reader. As far as the timing chain goes, they are supposed to last well over 100K miles.