Manufacturers spend billions of dollars a year refining their cars' on-board diagnostics systems. However, all of the sophisticated diagnostics in the world will do little good if you don't know what the warning lights mean.
Purpose
Modern automobiles have dozens of sensors to monitor engine, braking and chassis performance. Any deviation from the car's established limits will trigger a dashboard warning light.
Check Engine Light
Check engine lights can indicate failure of the emissions system, fuel injection system, faulty sensor feedback, overheating or lack of voltage, transmission failure, and engine knock ignition system failure.
Brake Lights
Almost all cars have two separate lights for the braking system: a red "ABS" (anti-lock braking system) light and "Brake Warning" light. The former indicates failure of the electronic anti-lock system, the latter a loss of fluid pressure in the brake system itself.
Dynamic Warnings
Many modern cars have traction or stability control systems that prevent and notify the driver of tire slippage. These lights can come in the form of amber "SLIP," "TC," or "TRACTION" lights (for spinning tires), or "ESC" (electronic stability control) lights for a sliding vehicle.
Safety Lights
Most cars have a diagrammatic warning that indicates a door that isn't all the way closed, a seat-belt warning light, and an "airbag disabled/enabled" light. Any or all of these will be red or amber.
Read more: Car OBD2 diagnostic tool
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