A malfunction indicator lamp (MIL), commonly referred to as the
"Check Engine Light" is an indicator of malfunction of the
computerized engine management system. It is found on the instrument console of
most automobiles. When illuminated, it is typically either an amber or red
On vehicles equipped with OBD-II, the light has two stages: steady
(indicating a minor fault such as a loose gas cap or failing oxygen sensor) and
flashing (indicating a severe fault, that will eventually destroy the catalytic
converter, such as a misfire).
When the MIL is lit, the engine control unit stores a fault code related
to the malfunction, which can be retrieved with a scan tool and used for
further diagnosis. The malfunction indicator lamp is usually labeled with the
text check engine, service engine soon, check engine soon, or a picture of an
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.
If the check-engine light comes on, here are some
tips on what you should do:
go for a state emissions test. In a late-model car, an illuminated "check
engine" light probably is a sure sign your car will fail the test. In some
states, it's an automatic failure, even if the problem was nothing more than a
loose gas cap. By the way, don't bother trying to fool the inspection station
by disconnecting the battery or using any other method to erase the trouble
code and turn off the "check engine" light. Your vehicle's computer
will let the inspection station know that its codes have been erased, and
you'll just have to go back again.
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. On some cars, a
yellow "check engine" means investigate the problem, while a red
"check engine" means stop right now.
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.
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.
the code read and the problem fixed. If you want to diagnose the
malfunction yourself, you can buy a scan tool at most auto parts stores.
Prices range from about $40 to several hundred, depending on the model and
the features. The tools come with instructions on how to hook them up and
decipher the codes. But unless you have a good knowledge of automotive
diagnostics, you're probably better off taking the vehicle to a
professional. Some automotive parts stores will read and interpret the
code for you without charge. Unless there is an easy fix, they may simply
refer you to a mechanic.