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Why
my Check Engine light comes on?
The engine computer or ECM
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All modern vehicles have a computer or the ECM (Electronic Control Module)
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.
How
this works?
Let's look deeper how all this works:
There is a number of sensors that provide the ECM with all necessary
inputs such as the engine temperature, ambient temperature,
vehicle speed, load, etc. According to these inputs, the ECM makes
initial adjustments adding or subtracting fuel, advancing or retarding
the ignition timing, increasing or decreasing idle speed, etc.
There is a primary (upstream) oxygen sensor
installed in the exhaust before catalytic converter that monitors the
quality of combustion in the cylinders. Based on the feedback from this
oxygen sensor the ECM makes further adjustments to the air-fuel mixture
to reduce emissions.
There is another, secondary (downstream)
oxygen sensor installed after catalytic converter in the exhaust that
monitors catalytic converter's efficiency.
Besides, there are few additional vehicle systems related to the
emission control.
For example, there is an Evaporative system (EVAP), that prevents
gasoline vapors inside the gas tank from escaping into the atmosphere.
The EVAP system also contains a number of sensors and actuators
controlled by the ECM.
The computer or ECM constantly tests operation of all sensors and
components. When any of the sensor signals is missing or out of normal
range, the ECM sets a fault and illuminates the "Check Engine" or
"Service Engine Soon" light also called MIL (Malfunction Indication
Light) storing the corresponding Diagnostic Trouble Code
(DTC) in the ECM memory.
The same happens if a mechanical component of controlled system fails.
For example, mechanical problem inside the transmission also can turn
the "check engine" light on. Even not properly closed gas cap will
cause the "check engine" light to come on - the ECM constantly checks
if the gas tank is sealed properly.
To sum up, when the "Check Engine" or "Service Engine Soon" light comes
on and stays on, there is a problem with your vehicle. This could be a
problem with the engine, transmission, or some emission-related
component or system.
The stored trouble code can be retrieved with the special scan tool by
the technician. The code itself does not tell exactly which part to
replace, it only gives a direction where to look for - the technician
has to perform certain tests specific for each code to find the exact
cause of the problem.
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