Tonight’s bedtime story is about a wicked, nasty little
fellow, known to all as the Check Engine light.
Now, you’d think that a light
would be good - illuminating stuff and helping folks out. But not the Check Engine light! This miscreant spent his days in a decidedly
un-illuminating way, spreading misinformation, hiding correct information, and
generally causing confusion and chaos throughout the land.
There are lots of misconceptions about the Check Engine
light (also called the CEL, which is what I’m going to call it from now on ‘cause
it’s easier to type) that we run into at the shop. I’m just going to quickly touch on a few of
them. The CEL - sometimes also called a “MIL”,
for Malfunction Indicator Light (not Mother In Law!) - indicates that
the car’s computer has detected a malfunction, and has stored one or more codes
related to the malfunction.
One thing we commonly encounter is the belief that the
code or codes tell you what’s wrong with the car. This is not correct! OBD II (which stands for On Board
Diagnostics, Second Generation) codes are nothing more than a symptom. The folks who built your car have written
diagnostic procedures for all the different codes. One of the first steps a
good technician takes is to refer to his diagnostic information system to look
up the procedure(s) for the code or codes in the car he’s working on. It’s a common occurrence for someone to swing
by one of the big chain auto parts stores to get the code read (usually with a
code-reader, not a true scanner). The
parts dude then sells the hapless motorist a part. Hapless motorist then happily skips off to
install the part at home. Imagine his
chagrin when the light comes back on a few miles down the road! This is because the cause of the problem was
never diagnosed – just guessed at.
I mentioned that there’s a difference between a
code-reader and a true scanner.
Actually, there are many differences – thousands of them if you’re
paying for one! The scanners we use at
the shop have many diagnostic functions in addition to the ability to read OBD
codes. They typically have
bi-directional capabilities as well as data-stream features, component tests, misfire
monitors, diagnostic databases, and many other powerful tools. Some have four-channel lab scopes and
multimeters built in as well. These
capabilities make diagnosing complex problems possible, where a simple
code-reader would leave a technician in the dark.
Sometimes, the cause of an illuminated CEL is not
really a big deal in terms of the actual operation of the car. For example, let’s say your gas cap is not
making a good seal against the filler neck.
This can trigger an evaporative emissions leak code. The car will run fine with this leak, so some
people will be inclined to just ignore it.
This is one of my pet peeves (I have an entire menagerie of them!) Sure, the small gas cap leak is not a big
deal, but you only have one light! If
something more serious happens, the light’s already on, so you won’t know about
it. Bad stuff can happen as a
result. Another reason that ignoring the
light is a bad idea is that many minor faults can increase the workload on
other components that are trying to compensate.
But ultimately, I prefer to repair all problems that cause the CEL to
come on because it’s the right way to do things.
The CEL tells us that the computer has detected a
malfunction – nothing else. At the end
of the day, maybe the poor Check Engine light was just misunderstood…
Thanks for your time!
Dave
DOC Auto
