My Engine Light Is On. Why?

All modern vehicles have a computer or the ECM (Electronic ControlModule) that controls the operation of the vehicle powertrain (the engine and transmission). The main purpose of this is to keep the engine running at top efficiency with the lowest possible emissions. With constantly growing demands for better fuel economy and new strictest emission regulations it's not very easy to achieve. The engine parameters need to be constantly and precisely adjusted according to various conditions such as speed, load, engine temperature, gasoline quality, ambient air temperature, road conditions, etc. That's why today's cars have much more electronics than in early days - there is a large number of various sensors and other electronic devices that help the vehicle computer or ECM to precisely control the engine and transmission operation and monitor emissions.
The vehicle computer system has self-testing capability. When the computer senses that there is a problem with some of the components it stores the correspondent trouble code(s) in its memory and lights up the "Check Engine" or "Service Engine Soon" light to tell you that there is a problem and your car needs to be looked at. To properly diagnose what is wrong, you need to take your car to a mechanic or a dealer. The technician at the dealership or a garage will then hook up the scanner to the car computer and retrieve the stored trouble code(s). Then he (she) will look it up in the service manual provided by a car manufacturer. The service manual contains the list of possible codes (about few hundreds) and describes what each code means and what needs to be tested. The code itself doesn't tell exactly what component is defective - it only indicates where to look, what engine parameter is out of normal range. The technician will have to perform further testing to pinpoint a defective part.

What do you need to do if the "check engine" light goes on
These tips can help you determine whether your vehicle has a loose gas cap or serious engine problems.

You're driving along in your car or truck and suddenly a yellow light illuminates on your dash telling you to check or service your engine. If you're like most car owners, you have little idea about what that light is trying to tell you or exactly how you should react.

Call it the most misunderstood indicator on your dashboard, the "check engine" light can mean many different things, from a loose gas cap to a seriously misfiring engine.

"It doesn't mean you have to pull the car over to the side of the road and call a tow truck. It does mean you should get the car checked out as soon as possible,

Ignore the warning, and you could end up damaging expensive components. It also can be a sign that your car is getting poor fuel economy and emitting higher levels of pollutants.

What the light means

The "check engine" light is part of your car's so-called onboard diagnostics (OBD) system. Since the 1980s, computers increasingly have controlled and monitored vehicle performance, regulating such variables as engine speed (RPM), fuel mixture, and ignition timing. In some cars, the computer also tells the automatic transmission when to shift.

When it finds a problem in the electronic-control system that it can't correct, the computer turns on a yellow warning indicator that's labeled "check engine," "service engine soon" or "check powertrain." Or the light may be nothing more than a picture of an engine, known as the International Check Engine Symbol, perhaps with the word "Check." In addition to turning on the light, the computer stores a "trouble code" in its memory that identifies the source of the problem, such as a malfunctioning sensor or a misfiring engine. The code can be read with an electronic scan tool or a diagnostic computer, standard equipment in auto repair shops. There are also a number of relatively inexpensive code readers that are designed for do-it-yourselfers.

Manufacturers originally used the OBD system to help technicians pinpoint and troubleshoot malfunctions. But the systems now are required under federal laws governing automotive emissions. Although larger trucks have been exempt from the requirement, that quickly is changing.

"The 'check engine' light is reserved only for powertrain problems that could have an impact on the emissions systems," says John Van Gilder, General Motors' lead OBD development engineer.

Exactly what the OBD system looks for depends on the make, model and year. The original systems varied widely in their capabilities. Some did little more than check whether the various electronic sensors and actuators were hooked up and working.

That changed by 1996, when, under OBD II regulations, carmakers were required to install a much more sophisticated system that essentially acts like a built-in state emissions testing station. The computer monitors and adjusts dozens of components and processes. For example, it continually samples exhaust emissions as they come out of the engine and again when they leave the catalytic converter, a device that removes carbon monoxide and hydrocarbon pollutants from the exhaust. The system also monitors your car's fuel system to ensure that gasoline vapors are not escaping into the atmosphere through a leak or even a loose or missing gas cap. In most cases, if a problem occurs, the computer will wait to see if it corrects itself before turning on the light. Modern OBD II systems are so thorough that state testing centers increasingly are checking for any stored trouble codes and foregoing the traditional tailpipe emissions test.

Some states are considering an advanced OBD system that would allow them to do away with emissions testing. If the "check engine" light comes on, the system automatically would send a remote signal to state officials, who would contact motorists who don't have the problem corrected within a reasonable amount of time. Privacy advocates are criticizing the idea as being too intrusive. Depending on the system, officials might be able to trace where the vehicle had been. Proponents say the system would free motorists from the time and expense of having to undergo annual or biennial emission testing, and it would help ensure that emission-related problems are detected and fixed more quickly. Oregonexpects to launch such a program on a voluntary basis in less than a year.

Remote diagnostics already can be found on GM vehicles equipped with the OnStar communications system. When the "check engine" light goes on, GM car owners can notify an OnStar representative, who can read the trouble code and provide advice.


If your "check engine" light illuminates don't react like one customer, who simply poured an extra quart of engine oil into her 2002 Toyota Corolla. Although extreme situations, such as low oil pressure or an overheating engine, might trigger a "check engine" light, your dashboard has other lights and gauges to warn you about those problems and probably a lot sooner. The best advice is to read your owner's manual beforehand and learn the purpose of the "check engine" light and every other gauge and warning indicator on your dashboard. Periodically, you also should test the "check engine" light and other dashboard warning lights. Usually, you can do this by turning the key to the key-on/engine-off position. Consult the owner's manual for more information. Replace any bulbs that aren't working.

If the "check engine" light illuminates, it will either blink or remain constant, depending on the problem. Either way, you should have the vehicle checked by a mechanic, although a blinking light indicates a problem that needs immediate attention. In late-model cars, a blinking light usually indicates an engine misfire so severe that unburned fuel is being dumped into the exhaust system, where it can quickly damage the catalytic converter, requiring an expensive repair. If that happens, you should reduce power and have the car or truck looked at as soon as possible. If the light is steady, the problem is not an emergency, but you should schedule an appointment as soon as possible. Today's automotive computers often try to compensate when there's a problem; so you may not notice deterioration in performance, even though your fuel mileage is suffering and your vehicle is emitting unacceptable levels of hydrocarbons and other pollutants.

"The customer is really, in a long run, potentially hurting their pocket book by leaving that light and ignoring it," says Jim Collins, a national training team leader for Ford Motor Company. In some extreme cases, the car's computer may reduce power for you, as it tries to limit the risk of damage.

If the check-engine light comes on, here are some tips on what you should do:

  • Look for a serious problem that requires immediate attention. Check your dashboard gauges and lights for indications of low oil pressure or overheating. These conditions mean you should pull over and shut off the engine as soon as you can find a safe place to do so.
  • Try tightening your gas cap. This often solves the problem. Keep in mind that it may take several trips before the light resets. Some vehicles have a separate indicator that warns of a loose gas cap before the condition sets off the "check engine" light.
  • Reduce speed and load. If the "check engine" light is blinking or you notice any serious performance problems, such as a loss of power, reduce your speed and try to reduce the load on the engine. For example, it would be a good idea to stop towing a trailer. Have the car checked as soon as possible to prevent expensive damage.