I had a "check engine" light on for a while, but the car was running fine, and I didn't bother getting it checked right away. Turns out it was my catalytic converter. I found out because I had to register my car in California and pass a smog test. Then my car failed the smog test because the catalytic converter failed the functional test.

So, word of advice, if the engine light comes on, get it checked right away! If I had, I would have saved $600. Honda extended their warranty to 8 years or 80,000 miles on the catalytic converter. If I had still been under 80,000 miles, it would have been a free replacement. Honda wanted about $1100-1200 to replace it. Got it done at Midas for half that!