The Automotive check engine light is a sensitive warning device. The clean air act requires the manufacturer to notify the driver when a problem exists that might be polluting the environment.
We all know the feeling we get when it comes on while driving. First, all the worst case scenario's come to mind and then in an effort to remain calm we tell ourselves maybe it's something minor.
In this quick article we'll discuss why the engine light comes on and some methods to turn it off. People tend to be less afraid of something when they understand it completely. This knowledge also gives you the freedom to make an educated decision on what to do next.
The warning light is turned on directly by the car's computer. Most manufacturers call it the PCM. This stands for power train control module. General Motors use to call it the ECM (Electronic Control Module).
You may find other names for this device, but basically it is considered the automobiles main computer. The PCM monitors the many different automotive engine sensors marking the high and low values and changes in this data stream.
It then divides the sensors into multiple channels. The computer monitors each channel and compares the data to specifications that are burned in memory.
service engine soon light is turned on, the computer is notifying the
driver that one of the channels it's watching is out of the specified
range. Keep in mind the flagged channel might have to go beyond the limits three times before the light pops on. This is to reduce false positives.
When this light comes on the car may have set a single code or multiple codes.
Lets use the oxygen sensor code as a common example of codes you might find stored in memory.
The range of operation the computer is looking for is between 50 and 800 mV. In this specific example the computer is also monitoring the changes in the voltage.
These changes in voltage are referred to as cross counts. This is a measurement of how often the voltage goes beyond and then falls below the 500 mv threshold.
If the oxygen sensor returns data that is out of the range of what is considered normal operation the check engine light is turned on to notify the driver of a problem.
Does this mean that replacing the oxygen sensor would turn off the light? The answer is, in many cases replacing the sensor will not turn the light off.
The reason the code is set in memory is because the sensor was detected to be out of range. Maybe the O2 sensor is faulty, but more than likely the sensor is operating correctly and is out of range due to an air fuel mixture problem.
A vacuum leak is an example of something that can cause this issue. This is where diagnosing the malfunction properly will save you time by avoiding replacement of unnecessary parts.
Using an automotive scan tool you can view the data stream for the particular sensors related to air fuel mixture like fuel trim or intake air temperature and look for problems.
Another rule of thumb when diagnosing an oxygen sensor code is to diagnose all the other set codes prior to the O2 one. This is because the exhaust stream malfunction can be triggered by other failures that effect the amount of oxygen in the exhaust.
To review, your computer is watching multiple channels and multiple sensors and is simply comparing the real time data with the parameters that have been programmed into the unit.
Above or below these data settings commands the PCM to notify the driver. Often these out of range readings can be either temporary or an intermittent car problem.
This is again where the scan tool comes in handy for figuring out why the engine light is illuminated. Specific codes point you to different areas of the computer controlled emission system that might need diagnosis or repair.
You can clear the codes and
run the vehicle and see if the service engine light comes back on. If it
does, then you have a path of diagnosis related to the codes that have
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One of the most popular automotive scan tools on Amazon is the Innova 3030g. This scan tool in my opinion is easy to use and covers diagnosis on vehicles from 1996 - present day.
This is not just a read and erase codes unit. It can also display individual sensor data. this is an older unit that will most likely be phased out, but for now it's a lot for a little.
There are more than 500 reviews from users of this scan tool on Amazon's website. This is a good way to learn more about this product and if it is right for you.
Click on the product link on the left to review the unbiased opinions of the general Diy auto repair public. Amazon's website only allows people who purchased and own the individual scan tool to wright a review. With their mass buying power they are able to offer it for less then many other retail automotive places and expedited shipping is available.
This next scanner is the 3140. This unit covers many popular domestic OBD 1 and most OBD 2 vehicles from 1985- present. You get more coverage but it is also more money.
This unit is not as popular, because of the price tag, but there are still more than 360 reviews of the scan tool at the time of this writing. It's very nice to have a tool that can interface with 25 years worth of cars.
One user did slam the product, because it would not scan his 1989 eagle talon. This is an odd ball vehicle. The Talon was an early mix of Chrysler and Mitsubishi technology. The tool interfaces with many, but not all cars and trucks from 1985 to 2015.
The original retail price
on the 3140 was over $450. Now it's selling for around $250
bucks. To me this makes it a good deal. But remember the Innova 3140
supports only Domestic obd1 vehicles. If your vehicle is 96 and up and
has a standard obd2 diagnostic port this tool works on both foreign and
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My information page on check engine lights and car code readers is one the most popular destinations on the car questions website. This is also one of my specialties due to intensive training and on the job experience. This next link takes you from the check engine light to auto scan tools.