how serious is the 'check engine' lite?

[7th gear]

i ask because obviously i had the warning lite display. i called my mechanic for an appointment and he told me it's probably fine and go ahead and keep driving. unfortunately, i had to do that as the fam was down a car to a tranny prob.

i drove the 986 to work for 3 days this week. the lite came on for 3 days straight. today i took it out and it didn't display

i'm still going to keep my appointment but i'm wondering if the scanner my mechanic talked about will be able to pull out what happened before the lite went out today. will the computer buffer the errors it gets in memory or history?

those of you who have made it thru this, was the diagnosis a 1-alarm fire or a 5-alarm fire???

thanks for the help!!