Service Engine Soon

Three times in the last six months the Service Engine light has come on, only to go out again before I take any action.  In each instance the light was on for a week.  I know that insufficient tightening of the gas cap can cause the light to come on, but it's not that.   Any other likely culprits?  Am I correct that there's no way to identify the problem unless the light is on?  That is, there's no history of codes stored, so the only way to diagnose the problem would be to take it in when the light is on? 

I think there is a history stored.

My GF's MINI had the light come on, we made appointment, before it could get in the light went out. It then went back on right before the appointment. Turned out to be the engine misfiring which triggered the light and then the problem would clear itself for some time and then come back. MINI said they were able to trace the original CEL to a misfire from reading the computer.

Do you notice and performance problems? She saw a drop in fuel economy even before the light went on and a change in how the car idled and left the line from a dead stop.

Long way of saying, yes, I believe there is a history.

Thanks for the ideas.  The plugs have only about 10k on them, but I'll take a closer look at the wires.

You could probably do better with almost any aftermarket "coil" if you will and wires.  Rusting of the #3 is a known problem, but I haven't heard why it happens.  If you get the factory replacement parts, they will just rust on #3 again (probably).  Don't know if the aftermarket stuff corrodes like that or not.

Actually I am dissembling my MINI today for a performance head install, and noticed that my aftermarket MSD coil is showing a bit of corrosion/deposits on terminal 3 as well, even though it's brass. Strange! I wonder if it has something to do with the angle of the engine and water gets up there or something?? 

It could be almost anything. Gas cap is one possibility. 

My shadetree mechanic (electrician?) days are mostly behind me.  I seem to be able to find time and motivation to work on my van, but when the MINI needs something, I generally pay someone else to do it.  Having owned a couple of Triumphs, I got a lifetime of experience and experiences in ten years' time.