In order to meet Federal Exhaust Emission and Fuel Economy regulations, today's vehicles are equipped with highly sophisticated electronic engine control systems. Your vehicle’s onboard computer receives information from a network of sensors and switches that convert engine operating conditions into electrical signals.

Based on the information and instructions stored in the computer program, commands are then sent to three primary systems:

• Ignition System
• Fuel System
• Emission Control System

Any of these commands can ultimately illuminate a “check engine” light. About 85% of vehicles on the road have either a check engine light on or some other safety issue such as a broken license-plate light, loose gas cap, a worn spark plug, poor emissions, low transmission fluid or cracked hoses and belts, according to the Car Care Council, a nonprofit consumer-education organization.

The first step to determine the problem behind an engine light is hooking up a scan tool or code reader to an On-Board Diagnostics connector that usually sticks out from under the driver's side of the dashboard.