Uncover the Mystery Behind Check Engine Light

Posted on 17. Jul, 2006 by in Saving & Investing

This post was contributed by my friend John Anderson who recently revealed to me the secrets behind the check engine light.

You are no longer driving the 1969 Ford Galaxy that you did in high school. That cherished old beater would quickly let you know of engine problems with an annoying knocking or clunking sound. Today’s cars are equipped with on-board diagnostic (OBD) computer systems that, among other reasons, monitor vehicle emissions.
Emissions? You could care less, right? Wrong! In addition to federal emissions standards, your car’s emissions are affected by many parts of the engine and the OBD system can let you know of problems from any of the vehicle’s major components.

So what does this mean for me and the bright engine light that is screaming at me? Well, a check engine light can signal a myriad of problems from a bad engine to a loose gas cap. Yes, the problem may be that you did not tighten your gas cap! Do not get you hopes up though as common check engine light problems include:

  • Faulty fuel mix
  • Engine performance problems
  • Electrical circuits
  • Drive train
  • Emissions sensors (ever heard of the O2 sensor?)

Not more that six months ago, I encountered this same problem with my 2002 Isuzu Rodeo. Shortly after we bought our SUV, we were driving home from a weekend vacation to visit the in-laws when the check engine light blinked on. Instantaneously, I envisioned thousands of dollars going to the pocket of the next greasy mechanic who would be replacing our engine.

I took our car to a local mechanic and he said that he could check what out our car’s OBD was telling is for $20, and then turn off the light and see if it returns. The greasy mechanic met me outside and hooked up his OBD tester to my car. A grim smile came to his face, and I feared the worst. He chuckled, walked back to the rear of my car, opened my fuel door, and tightened my gas cap! Mechanic Joe then said, “That should do it. Tell your wife to make sure the gas cap is tight.”

I was both relieved and angry at the same time. Relieved that I would not be writing a $4,000 check to replace my engine and angry that such a small problem had gotten me into such a tizzy. What is next? Will the light also go on if I run low on windshield washing fluid?

With that experience under my belt, I encountered the same problem a week ago when my wife called me on her drive home from work and said that the check engine light was on again. My first thought was, “Honey, did you tighten the gas cap?” I did not want to pony up $20 again for mechanic Joe to tell me the same thing, but I still wanted to make sure it was not something big.

Searching the internet for some shortcut, I found that AutoZone will check your car’s OBD for free and make sure the problem is not trivial. I called the local AutoZone to make sure they participated in the program and they confirmed the report. I asked if the guy could also turn the light off, and he said that he could not, but between him and I, that I could just unplug the negative battery terminal for 30 minutes and the OBD system would reset.

Thankfully, I saved myself $20 bucks, found out that the problem was minor, and avoided the greasy mechanic. If you happen to be in a similar situation with your check engine light, I recommend:

  1. Take your car into AutoZone and have then tell you what the OBD code means. If the problem is major (i.e. transmission, engine), take your car in to be repaired by a trusted mechanic.
  2. If the problem appears to be minor, disconnect your negative (-) battery cable to reset the light and wait to see if the light returns.
  3. If your light turns back on within a short period of time, take your car in.
  4. If the light does not come back on, thank your lucky stars and be glad you avoided being ripped off by a greasy mechanic.

Other check engine light resources include:


4 Responses to “Uncover the Mystery Behind Check Engine Light”

  1. mapgirl

    17. Jul, 2006

    I don’t know about your wife’s car, but my car has a gas cap that clicks when it’s on properly. If you over tighten, it will make more clicky sounds. But I always know that my cap is on tightly with this kind of cap. You could probably find an aftermarket cap like this one to put on so your wife always remembers to tighten the cap correctly.

    Now, ask me if I remember always to shut the little door to the gas cap! 😉

  2. Ryan

    20. Jun, 2007

    Thats the problem when your truck or suv are 10 years older, it starts to show signs of problems. For me, i have my problem solved with the help from here http://www.suvandtruckparts.com. Thanks Guys!

  3. Lynn

    27. Apr, 2011

    Same issue with the intermittent light. Had it checked even though light wasn’t on when mechanic ran the OBD; turned out that an EGR valve needed replaced. I avoided the greasy mechanic who wanted to charge me $400 for a $100 part with another $100 for labor on something that it took my bother 5 minutes to fix with a part I ordered (he also cleared the computer code)..total cost, $138 and a beer for my brother. I drive 460 miles back home a day later with intent to run car through emissions testing. Waited less than 24 hours; the check engine light came back on again.


  1. Mapgirl’s Fiscal Challenge / Articles I liked this week - February 21, 2007

    […] Beancounter Blog on the mysteries of the check engine light. Given my 98K mile service recently, I think this was an interesting post. I admit, the check engine light had been going off in my car intermittently for the past month. Just when I think I was going to call the dealer and take it in, it would turn off. It was quite odd. Finally, my friend thinking of purchasing my car is what pushed me to take my car into the shop. Good thing too since I didn’t know my clutch needing replacing quite so bad. If you’re wondering when or what to do when your car needs maintenance, try Flick & Flack, the Tappert Brothers from Car Talk on Saturday mornings on NPR. (ooh ooh! The DIY guide!) […]

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