"Check Engine Light" Myths

Just browsing the internet and came across this article. Found it because someone was going on about how the only reason they're MIL came on is because he hit 10,000mi and wanted to know how to reset it without going to
the dealership. So anyway, talks about a couple of myths, here's the cut-and-paste:
------------------------ Many people are confused about the check engine light and why it comes on. We thought we would address some of these common myths about why your check engine light comes on and clear up some of the fallacies about it.
Myth 1: The trouble codes will tell you what sensor to replace. This is an expensive frame of mind. The Check Engine Light directs your attention to a problem. A trouble code description then directs you to a circuit or system. It will not tell you what sensor to replace. It is about as vague as stating that a book is in the automotive section of the library. You now know what section to go to, but no clue where to look yet. That is why there are flow charts or trouble code trees and diagrams to guide you through specific tests to determine what is the problem. Often it is a broken wire, loose connector or some other cause, than the sensor itself.
Myth 2: You can clear the codes by disconnecting the battery. This is true on pre-96 vehicles and very few, if any, OBD2 vehicles. Some folks will say, "I disconnected the battery for 15 minutes and the light went out, so it cleared the codes". No, it didn't. It may have reset the ECM and the light is no longer present, but the code is still there and if the problem has not been repaired, the light will come back on. The next time you have a problem, now you or the mechanic who is working on the vehicle are going to have to contend with that code as well as any other that is present.
Myth 3: When the check engine light comes on, it always means you have to replace something. One of the first things that needs to be done when diagnosing the check engine light is to clear the trouble codes, road test the vehicle and then recheck the trouble codes. If the codes come back, then start with the lowest number code and go through the flow charts and diagrams.
And the last Myth: The Check engine light means an O2 sensor problem. Anyone who has taken this to be true and has spent quite a bit of money on replacing oxygen sensors knows this is not true. First, you will not know, nor will anyone else, what the problem is until you have the trouble codes. Even if it is an oxygen sensor code, alot of times there are other causes for the code to come about.
Vacuum leaks, poor fuel quality, low or high fuel pressure, a compression problem or a plugged catalytic converter could cause it. If the oxygen sensor is bad, then replacing the sensor still does not finish the repair. If an oxygen sensor failed, then there is a problem with the emissions of the engine. Usually when an oxygen sensor fails, it is because it has become contaminated. Contamination caused by an engine that is not running properly. So you will still need to determine where the originating problem came from. More often than not, there will be no trouble codes for that problem and it will not be evident without some specific tests.
We hope that this will clear up some "myth-conceptions" of the check engine light, because we have seen many people all over who have been telling folks that it is on because of this or that. We've tried to locate where it is that they buy their crystal ball, but alas, we have yet to find a reliable crystal ball. Until then, we rely on good old common sense, a good scan tool and a quality information system, like Alldata. ------------------------
Source: http://www.troublecodes.net/articles/myth-conception.shtml
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Steve, now you have eliminated 1/2 posts here if everyone reads this!!

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Did I win?

on.
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Absolutely!!

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While I agree with most of the myths part of myth 3 I totally disagree with. That is the first thing to do is resetting the code and see if it comes back. The code and information available in the computer should be analyzed to determine if the problem can be fixed without sending the customer out on the road with a problem that could put his/her safety at stake. The OBD2 system has a lot of information and a lot of marginal circuits causing intermittent checks can be diagnosed without the need to sluff the customer off by resetting the error.

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Woody wrote:

Well, you are wrong. If you can diagnose intermittent problems consistently, you need to come apply at our dealership....we will pay you well over 100K dollars a year, if you can actually live up to your claims. Hell, it would sure take the load off the rest of us!
We work with intermittent problems. If the problem is not occuring when you have the vehicle, its impossible to say for sure what the problem is. Yes, you can take an educated guess, and I often do. But it's still just a shot in the dark. It's either that, or kick the vehicle out until the problem is there consistently.
Ian
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That is the expected answer for any warranty problem from the dealer. Reset the light and kick it out if the light doesn't come right back on. No matter it may take several drive cycles for the tests to complete and the problem to show back up. Unfortunately the service side is still working in the past and isn't stepping up to the wealth of information and diagnostic ability available today. While I respect your knowledge and help here I respectfully disagree with you on this.. I am not advocating throwing parts at codes as this is futile, I am advocating analyzing the information available from the computer and checking components that could cause the problem.

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I'm going to have to also disagree with you Woody. Some codes take a few drive cycles to turn the MIL off. If you automatically assume the problem is there, you are going to be wasting customer money diagnosing an intermittent. I can point out a perfect example of another reason to first reset and roadtest.
My ABS light came on a few weeks ago, when I check the codes, you wouldn't believe it if you saw it yourself. No less than 10 codes in history and 4 on the current ignition cycle. WTF? Something must be really wrong, one of the codes was DTC 55 - EBCM Malffunction. A quick reset and road test revealed only one code, DTC 35 - RR Speed Sensor Open or Shorted to Ground or BAT+.
An intermittent can cause multiple codes and some DTCs take a few ignition/drive cycles to turn the MIL off so the MIL may be on but the problem not there. Two excellent reasons to reset and road test.
IMO,
Steve

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Many times, the diagnostic procedure instructs to clear the code and test drive to see if the code returns. When it doesn't, then what?

Exactly. Or are you suggesting that the vehicle be relinquished for however long it takes for the problem to re-appear?

Feel free to post any pointers to where one might find these magical tools and information data base.

And when the freeze frame doesn't show anything and the examined components check out okay?
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Amen to Steve and AAR, go through this everday, Steve , ran into the same abs problem on my Lumina, Snappy scanner pointed to ebcm, doing a road test revealed lt ft speed senor dropped out before the others when slowing down. that fixed all the issues.
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Hey Shep, are you currently using he brick (MT-2500)?
If so, are you using the VCI cartridges?
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Woody wrote:

Woody, are you a GM technician working at a dealership? Because if you aren't, then I will respectfully say that you have no idea what you are talking about. If you have an intermittant problem with a history code and nothing is happening at the time, you can check every component in the car, it will all test out fine. As far the hard codes go, I believe that Steve gave you an excellent example of a problem that needs the reset and road test before moving on. I've run into the same scenario that he described.
You may be thinking of evap codes. In those cases, yes, I'd agree that you might want to do some testing. But since 75% of all evap codes are related to the customer leaving the gas cap loose, it makes sense to us (because we've seen it a million times) to tighten the gas cap, clear the code, and tell the customer to drive it and bring it back "if" the light comes back on.
Ian
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