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!