The Check Engine light, which is officially called the "Malfunction Indicator Lamp" (MIL) alerts you when your vehicle's OBD II system has detected a potential emissions problem. Depending on the nature of the problem, the Check Engine lamp may come on and go off, remain on continuously or flash. Of course, none of this gives you any clue whatsoever as to what might be going on.

Some people panic when they see the light, fearing their engine is experiencing some kind of major problem. But fear not, because in most instances, the problem is usually minor and is nothing that requires your immediate attention.

Here's how the Check Engine Light works. When the OBD II system detects any fault that may cause an increase in emissions, it sets a "pending code" in the computer's memory. The Check Engine Light doesn't come on yet because the system needs to make sure the problem is real and not a temporary glitch. If the same problem occurs on a second trip (under the same driving conditions), the OBD II system will then set a diagnostic trouble code (DTC) and turn on the Check Engine Light.

NOW WHAT?

If your Check Engine comes on, what should you do?

If no other warning lights are on (temp, oil pressure, charging, etc.), AND your vehicle is driving normally (no unusual sounds, smells, vibrations, loss of power or other signs of trouble), you don't have to do anything immediately. But you need to find out why the light came on when it is convenient to do so.

The only way to know why your Check Engine light is on is to connect a scan tool or code reader to the OBD II diagnostic connector under the instrument panel and read out the code. If you do not have a code reader or scan tool to do this yourself, you can take your car to an auto parts store for a free diagnosis (Autozone & Advance Auto are currently offering this). If you can't get a free diagnosis at an auto parts store, you will have to take your vehicle to a repair shop or new car dealer for a plug-in diagnosis. Be warned that this is usually expensive. Most charge $75 or more to perform this service. For the same money, you could buy a code reader or basic scan tool and do it yourself.

To connect 1 or more diagnostic computers to your car  to communicate with the various on-board computers for systems such as Engine management, ABS braking systems, Traction Control, Airbags, Instrument panel, Central locking, Windows, Wiper control, Lighting control, Body computers etc etc. in order to interrogate those systems about possible stored fault codes in memory, to check live data values, input & output signals from various sensors. To check operation of various actuators, re-program systems where possible.

Many people are under the impression that you can simply plug in a diagnostic computer and it will tell you what is wrong BUT, there is more to it than meets the eye!  Diagnostic systems will give the technician some indication of what components were affected by a fault condition but that does not mean to say the problem lies with that component. For example:- a Vauxhall engine puts up a fault code for an air flow meter, but the actual fault is more likely to be simply a dirty throttle butterfly flap and can be cured by cleaning without the expense of an air flow meter at £100. Some cars also put up fault codes for lambda sensors in the exhaust when in fact it is the airflow meter which is faulty!! Owning a diagnostic tool doesn't turn a mechanic into an electronics expert! Knowing how to use the tool properly is an art in itself!

Hence the necessity to investigate in depth the live data information before jumping to expensive incorrect conclusions.