The "check engine" light tells you when you need to take your car to the shop... in theory. In reality what it does is flip on any time the On Board Diagnostics (OBD) detects and error. In many case said show will charge you just to plug a simple, hand-held device up to your car that will display what the error actually is. That's what is know, colloquially, as a rip off.
Why is it that newer cars, even cheap ones like the Honda Fit, can display an odometer reading, remaining oil-life, fuel economy, and details about what song is on the radio but can't display an error code for the driver to read? Sign the petition here to end the Check Engine light in favor of just having cars actually display their errors on the dash.