The service engine soon or check engine warning light is your cars friend but it
can be very sensitive and annoying. In this quick article I will discuss why the
check engine light comes on. I will also touch on a few ways to diagnose codes
and turn off the amber check engine light.
The check engine light is
turned on directly by the vehicle computer. Most manufacturers call this the
PCM. This stands for Powertrain Control Module. You may find other names for
this unit but basically it is considered the vehicles main brain.
The
PCM watches many different engine sensors. It then divides these sensors into
multiple channels and monitors each channel and compares the range of data to
the specifications that are in memory. The pcm is monitoring both inputs like
coolant temperature and oxygen sensors but also output controls like idle air
control motor steps and egr percentage open or closed.
So the amber
engine light is turned on when the computer notices a parameter out of range.
Keep in mind that output control parameters like the EGR and IAC mentioned above
are usually measured with a resistance value or reference voltage to assure
proper component operation. And also the check engine light is amber to express
caution not red like the oil light that means emergency or turn off the vehicle
now or damage your engine.
When the check engine light is turned on the
computer is notifying the driver that one of the channels that it is monitoring
is out of the specific range. When this light comes on the car may have set a
single code or many codes.
This is where diagnosing the check engine
light properly will save you money and time in replacing unnecessary parts.
Using an automotive scan tool, you can view the data stream for the particular
sensors that set the code.
Another good rule of thumb when diagnosing
sensor codes is to diagnose the failures that are set with lower numbers, prior
to diagnosing the other sensor codes. This is because the higher number codes
could be set by the lower number failures. This is not a hard rule and you may
have to apply some common sense to this as in a misfire code should be addressed
before repairing a rich exhaust code. Since the UN burnt fuel from a cylinder is
detected as a rich condition.
To review your computer is monitoring many
channels and multiple sensors and simply comparing the data with the parameters
that have been programmed into the PCM itself. Above or below these data
settings commands the PCM to notify the driver they are polluting the atmosphere
by turning on the service engine soon light.
In some cases these out of
range readings can be either temporary or intermittent. This is again where the
scan tool comes in handy for figuring out why the check engine lamp is
illuminated. You can clear the codes and run the vehicle and see if the check
engine light comes back on. If it does, then you have a path of diagnosis
related to the diagnostic codes that have returned.
If the warning light
stays off after you clear the code then don't worry about it. This could have
been an isolated glitch or an intermittent problem. Also note that some times
carmakers change the parameters in the computer memory so a particular code will
not be so sensitive. When this happens the manufacturer will issue a technical
service bulletin to notify mechanics.
Mark is a Certified Master
Technician that provides automotive repair
advise for do it yourself auto mechanics.
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