so i've been having issues with my service engine soon light coming on. the dealership replaced an o2 sensor they thought was the culprit (the code they found was for a cylinder misfire, which i know is a generic code that could mean any of a number of things).

the very next day the light came on again, stayed on for 3-4 days, then spontaneously went out before i could take it back in. it stayed off for around 2 weeks then came on again the other day. today i took it in, they checked it out and said it doesn't have any new codes in the data recorder (or whatever they called it). the light was on, but the car apparently wasn't saying why

2 different techs road tested it and couldn't find anything wrong. the car seems to be running fine, but the idle seemed a bit high (1000-1100 rpm) when i took it in. now that the light is cleared it's down to 750rpm. granted, it was warmer this afternoon than it has been the past few days...would that explain the variation in the rpm at idle?

anyway, my question for those better versed in this sort of thing than i am: is it normal for the service engine soon light to illuminate without there being a code thrown to go along with it?