Computer diagnostics. Even a hand held, relatively good scanner can't do everything that's needed on modern cars these days. If you drive a new car and your check engine light comes on, there's basically no choice but forking over some cash to the pros with factory diagnostic software.

All is well and good with doing your own repairs if—and this is a big if—you're ok with driving a (much) older car. If you want something, say, model year 2000 or later (give or take of course) there's not much that can be done without expensive computer diagnostic equipment.

Know to pull over and stop when then oil pressure light comes on or the engine overheats. I've seen far to many motors toasted because "it started overheating last week and I only got a chance to bring it in now."