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Check Engine Light

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Ah, that nasty "Check Engine" or "Service Engine Soon" light.  When it pops on exactly what does it mean?  Well first of all it doesn't mean "panic" although that seems to be the basic reaction for a lot of people. I've had a number of people confide in me that upon the check engine light coming on that they immediately contacted their dealer and made an appointment to come in and have it "fixed".  I have been told that people paid as much as 79.00 simply to have the car "put on the computer".  I have also been told by many people that since they didn't notice any problems with the car that they just ignored the check engine light.  Personally from my own experience neither extreme is the right answer.

First, simply ignoring the thing is wrong.  There are so many things that can cause the check engine light to come on that at a minimum you have to know what the underlying condition is that caused the light to come on.  Once you have that knowledge you can then decide as to a course of action.  On the other hand, paying anything to get the "code" read is generally not necessary.  My local Auto Zone store will gladly do it for nothing.  They'll even provide some guidance on the course of action to take to address the issue.  Of course, if you're a vehicle repair enthusiast like me you can buy your own code scanner for under 100 dollars.  I say "code" because that's what you're going to get - a code.  One of the great things about the internet is that armed with a code there is no end to the information you can quickly get about the cause, potential concerns and how to repair it.  Beware of advice from the internet however.  There is plenty of great advice out there but there is also some horrible advice too. 

Not all codes are created equal either.  A friend of mine asked me to check out her car after the check engine light came on.  Her problem - she didn't tighten the gas cap.  That's right, not having the gas cap on securely caused an error to occur in the fuel system which illuminated the check engine light.  Using my code scanner I was able to read the code as well as reset the light (after tightening the gas cap!).  Can you imagine spending money to get that "diagnosis" somewhere?  Ouch!

One thing I've discovered is that it really doesn't take that much to cause the system to illuminate the check engine light.  Recently the check engine light came on on the Project 300K Subaru.  I hooked up my code scanner and found the P0420 code (see above pictures).  Translation: Catalytic System Efficiency Below Threshold.  Translation of the translation:  There are two oxygen sensors on the car, one before the catalytic converter and one downstream.  To get this code, the downstream oxygen sensor reading compared to the upstream sensor reading is off.  There are four possible problems; a leak in the system, a bad upstream oxygen sensor, a bad downstream oxygen sensor or a bad catalytic converter.  It would cost about 700 bucks to replace it all (assuming I can do the work and I'm not too sure about replacing the converter) so I figure the better part of a grand.  Well, I'm not about to drop a grand unless I have to.  I did what I call a "hard reset" by disconnecting the battery for a while.  That extinguished the light.  It stayed off for several weeks before it popped on again.  I checked the code - yep - P0420.  After another "hard reset" I went a month before it came on.  I monitor fuel economy and there has been no change.  One thing I have considered is if I am somehow getting fuel with something in it that may have an effect on the oxygen content of the exhaust.  Regardless, I am not going to spend hundreds of dollars on what seems to be a transient situation.  I know this may sound strange, but every time the light has come on I was in the exact same location on KY-155 near the Spencer/Jefferson County line.  Deep down I think there is some electromagnetic disturbance there.

I recently got the code P0335.  Here's where checking it out pays off.  This is the code for a problem with the crankshaft position sensor.  If you've read the rest of this blog you know that I've had some troubles with the engine wanting to stall out at idle after driving a while.  It turns out that a faulty crankshaft position sensor can cause this.  Since I don't have the diagnostic tools necessary to really solve this, I took it into my dealer.  They figured out that there was a bad connection at the ECM and "fixed" it for under 50 bucks.  I put fixed in quotes since unfortunately after running well for some time the condition returned intermittently.  I am certain that there is still a loose connection somewhere but I refuse to pay anyone who won't 100% guarantee that they can isolate and fix the problem (and no one will make that claim). 

Here's my advice - and remember that's all it is - advice - If the check engine light comes on you must find out as much as possible about why it came on.  Since it could be anything from a loose gas cap to a potentially serious problem you can not just ignore it.  Once you understand the problem and potential solutions you can then make the best decision regarding your course of action.  If you are uncertain, you do need to enlist the help of a professional automotive technician to resolve the problem. 

This page was last reviewed on July 29, 2014.

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