Why Is My Check Engine Light On? The 5 Most Common Causes

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Is your service engine soon or check engine light illuminated? Does it come on briefly and then disappear? Does it blink or flash and then go off, or come back on and remain on for extended periods of time? The fact that you have a malfunction indicator lamp that is illuminated could be telling you that something is very wrong. Unfortunately, it could also be a reminder that you are costing yourself money through reduced fuel efficiency by putting off the inevitable. After all, if you reside in an area where annual Federal emission testing is mandated, then it is only a matter of time until your “Service Engine Light Woes” must be addressed.

#5 – The fifth most likely cause of your glowing service engine soon light is probably a cylinder misfire. Designated with trouble codes 300 through 312, these types of codes usually occur around the 100,000-mile mark and eventually involve periodic replacement of all the ignition components or a good preemptive tune up (recommended). Spark plugs, ignition coils, and ignition coil boots can all be listed as common causes of these ignition codes.

#4 – The intake air temperature (IAT) code differs from other codes on our list in that it is typically caused by man. When the air box is disturbed, for purposes of air filter element replacement, the IAT connector is often left disconnected, causing an inadvertent code to be set and a service engine light to be illuminated.

#3 – Late model oxygen sensor heater circuits are yet another common cause of the annoying service engine soon lamp. The good news is that some manufacturers have “smartened up” and began to utilize fuses in an effort to spare costly sensor heater elements (not that it always works out that way).

#2 – This is the most valuable tip of the article: It is right here, do you see it? *Just because the code description has the words “EGR valve” in it, doesn’t necessarily mean that replacement of the exhaust gas recirculation (EGR) valve will be required. In fact, it seldom is the cause. Try faulty vacuum or exhaust lines and hoses, vacuum solenoids, or clogged EGR passages in the intake.

#1 – Easily taking the top spot is the evaporative emissions system leak (large). This code can be caused by failure to tighten the fuel filler cap tightly.

S.M. Darby

S.M. Darby

I am a freelance author with over 25 years of experience as a professional, ASE certified automotive technician and shop owner, muscle car enthusiast, avid street racer, and classic car restoration specialist.

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