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What Causes My Check Engine Light To Come On?

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If your vehicles check engine light has ever come on and you would like to learn the basic causes then give this a quick read. The first and most general answer is, the check engine light will come on anytime your vehicle is potentially causing more pollution then allowed. This many times progresses into drive-ability and fuel mileage complaints as well.

I have been an automotive tech for the last 14 years,  8 years of them spent diagnosing complex BMW engine management systems. My goal is to provide some general information to clear up the most common misconceptions about why check engine lights come on. I will also discuss how to clear the light toward the end.

CEL

What is OBD II?

All vehicles model year 1996 and newer have what is called OBD II or On Board Diagnostics systems. The system illuminates your vehicles check engine light and stores a fault code any time it sees a problem that may cause increased pollution into the atmosphere or your vehicle to run improperly. This is something standardized across all manufacturers to help control pollutant emissions into the atmosphere. OBD II also allows for diagnostics of the vehicles emissions control devices via a test port under the dash. This port is called the DLC (data link connector). In general a hand-held code reader is connected to the test port and used to read out the fault codes stored in the system. Code readers have become much more user-friendly for the do-it yourself type and basic units can be purchased online for as little as $20- $100, (I have included two links at the end of the article to give you a better idea what they are). These code readers are also the device used to clear your check engine light!  

So Why Is My Check Engine Light On?

This particular warning light, also known as "service engine soon" or MIL (malfunction indicator light), will come on when the vehicle has run specific self checks and the tests have failed. The system generates DTC's or diagnostic trouble codes. These fault codes are what I can read out with my code reader. Here are some of the self tests performed as the vehicle is being operated:  NOTE: these are also my top five answers for, "what causes my check engine light to come on?".

  • Your vehicle tests the fuel tank for hydrocarbons, (fuel vapor), leaking into the atmosphere. The most likely cause of test failure in my experience is loose, missing or leaking gas caps. Check your gas caps and save the environment and a repair bill!
  • The engine computer monitors the content of the vehicles exhaust via devices called oxygen sensors. These sensors check both the raw exhaust and the exhaust after it is cleaned by the catalytic converter. Oxygen sensors are a relatively common failure and in my experience if they are setting fault codes %90 of the time the oxygen sensor needs to be replaced.
  • The catalytic converter is tested by comparing the readings of the oxygen sensors from before the cat converter and after it. There is a threshold that cannot be overshot or this will also cause a fault code and illumination of your vehicles check engine light. This is another item that can set a pretty straight forward fault. Depending on your code reader it will say something along the lines of, " catalytic converter efficiency below threshold", and this would be another %90 indications of a failed catalytic converter.
  • The engine is monitored for misfires. In general a misfire occurs when one or more of the cylinders does not completely burn all the fuel in the combustion chamber. The result is an engine that runs rough and in some severe cases may cause stalling and a no start condition. The engine computer will store faults for the particular cylinder/cylinders that have misfired. Once the faults are read with your code reader I usually inspect the spark plugs of the faulted cylinders. Spark plugs would be the number one cause I find for misfire faults causing service engine soon lights.      
  • Tests of various engine sensors are performed and faults set for each. One common problem would be a faulty or dirty air mass meter. This device measures the air as it enters the engine. Fortunately most air mass meters are easily accessible and some even easy to replace.

 As I mentioned before these are my top five reasons. I feel the common misconception is that there are only a handful of things that could cause the warning light to illuminate. Truth is there are many other causes and a large list of fault codes that can be generated by the system and read out with your code reader.

How Do I Turn Off My Service Engine Soon Light?

 Now that you have a better understanding of the cause for check engine lights you can see how important being able to read the vehicles diagnostic trouble codes is. It is near impossible to just guess the cause without knowing the code that the system has stored. If you're the "do it yourself" type and enjoy learning new things then a hand-held code reader is an awesome thing to have in the toolbox. If your fault code is for something basic it will save you hundreds in repair costs.      

Keep in mind after the cause of your check engine light is resolved you will want to clear the fault with the code reader. In theory the vehicle will run the associated test and when it sees the problem is resolved it should put the light out, (perfect example is tightening your loose gas cap). You can try this but in my experience it is hit or miss and frankly its just easier to use the tool!  

Here are two popular options of code readers. Both work awesome!  

Autel MaxiScan MS300 CAN Diagnostic Scan Tool for OBDII Vehicles
Amazon Price: $39.99 $15.99 Buy Now
(price as of Sep 23, 2014)
This is a great low budget unit. I use it daily for quick code reads or to clear trouble codes.
Actron CP9575 AutoScanner
Amazon Price: $187.99 $99.99 Buy Now
(price as of Sep 23, 2014)
This unit really does do a lot for the money! If your a little more serious about automobiles this is my choice.
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