It's usually accompanied by a OH NO feeling in the pit of your stomach. The light could mean a costly problem, like a bad catalytic converter, or it could be something minor, like a loose gas cap. But in many cases, it means at minimum that you'll be visiting the car dealer to locate the malfunction and get the light turned off.
The Check Engine light — more formally known as the Malfunction Indicator Lamp (MIL) — is a signal from the car's engine computer that something is wrong.
Prior to 1996, carmakers had their own engine diagnostic systems, primarily to ensure their cars were compliant with Environmental Protection Agency pollution-control requirements. Starting with model-year 1996, automakers standardized their systems under a protocol called OBD-II, which stipulated a standardized list of diagnostic trouble codes (DTC) and mandated that all cars provide a universal connector to access this information. It's usually located under the steering column and is easy to access.
Deciphering the Code
The Inspectors can buy inexpensive code readers that connect to this standardized onboard diagnostics (OBD) port and search for the code's meaning on Web sites such as Engine Light Help. The Check Engine light can even be turned off by some code readers, even though this action alone does not actually repair the underlying problem. In many such cases the light will simply come back on later.
Experts say that many drivers confuse the "service required" light on the gauge cluster for the Check Engine light. These warning lights are unrelated. The service required light just means the car is due for an oil change or other routine maintenance. It is not the indicator of trouble that the Check Engine light is.
Check Engine lights come in orange, yellow or amber, depending on the manufacturer. If the light begins flashing, however, it indicates a more serious problem, such as a misfire that can quickly overheat the catalytic converter. These emissions devices operate at high temperatures to cut emissions, but can pose a fire hazard if faulty.
If your smart phone is a Apple product Look in the itunes store under OBD Car Doctor or Fuzzy Codes, there are a few available. Look at the Google play store if your phone is a Droid there are a few free ones available there.