[sd_boxster]

No garage is decently equipped without an OBD-II scanner. I personally cannot STAND not knowing why my MIL light is on (thought I've not yet thrown a code on the Boxster).

If if it always coincides with fueling, your gas cap does sound like the likely suspect.

Many trouble codes will self-clear after a number of drive cycles once the condition has been corrected - so it may not be true that you need to use a scanner to clear that code. Of course, knowing what code you're throwing is the important part - it's the bit of information that will tell you "safe to ignore" or "call a towtruck".

I inadvertently switched the IAC and TPS sensor harnesses on my last car after a particularly harrowing weekend of modding. Without a code scanner, I would have freaked out and spent hours tearing into my motor...