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
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
Myth 3: When the check engine light comes on, it always means you have to
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.