ecu code

my friend has a 99SE-L and his computer threw a code 0901....which came out to be a rt front pass o2 sensor heater or something like that. we then reset the ecu and the CEL hasn't come back on yet. anyone have any idea if he should still get the car checked out by nissan or is it okay for now. i mean i know that the code is not being thrown again by the lack of an engine light, but what would trigger it to go off in the first place?

my new 02 max spit out a code concerning an automatic tranny for some reason. i reset it and it hasn't come back on. funny thing is i have a 6speed. i wouldn't worry about it if the light is off