I was just saved by my Scan Gauge today.
I was on a trip, a hundred something miles from home, when my "service engine soon" light came on.

This was in my year old 2005 X.

Great. Do I ignore it and drive home? Do I check it out immediately? Do I stress out about it?
Nope - Scan Gauge showed Code P0455. There was a Nissan dealer near by. I asked the girl at the service desk if they could tell me what the code meant. I heard a stressed sounding mechanic say "we can't take him right now". Service girl said - "no no no - can you just look up code P0455 and tell me what it means?"
He could handle that apparently - Turns out it was just the stupid loose gas cap thing.
I just reseated the gas cap, cleared the code with the scan gauge (maybe it would have cleared on it's own), and went on my way.
COOL!!!

Without the scan gauge, i would have waited around that dealership for hours just to find it was a loose gas cap, or drove 2 hours home learing at the "service engine soon", wondering what the heck was wrong and leary of impending doom.