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The CEL. (That’s what mechanics call the “check engine light.”) The folks over at Lifehacker recently posted a list of “the five most common causes of a check engine light and what you should do about them,” and it’s not a bad list. Check out the article if you wish, but I’ll save you some trouble. The list: faulty oxygen sensor, loose or faulty gas cap, faulty catalytic converter, faulty mass airflow sensor, bad spark plugs and/or wires. They suggest taking your car in for diagnostics (which will work, of course), but most Toolmongers know by now that with a simple reader you can at least see what the computer thinks is wrong. And if you don’t own a reader, you can generally rent one (or borrow one with the purchase price as security) from auto parts stores.

A few additional comments beyond Lifehackers, though, from the experience of one who’s suffered through most of these failures:

Replacing an oxygen sensor is generally dirt simple, usually involving just unscrewing the old one and replacing it with the new one. They’re sometimes a little difficult to get at, and you want to be careful about banging the replacement around when installing it. Otherwise, it’s no big deal. Definitely not worth paying a couple of hours of book rate at the shop if you’re tight on cash. Just make sure you get the correct sensor when you order the replacement. Hint: Take the old one with you when you go to pick up the new one, if possible. If not, snap a camera phone photo of the old one.

Yes, a leaky gas cap will indeed screw you over in emissions testing. If you have to replace yours, there’s nothing wrong with buying a cheap-ass version from the auto parts shop, but make sure it’ll fit under the door flap before you purchase. And just skip any locking version, or permanently unlock it and ditch the key. You’ll lose it eventually and get stranded.

In terms of replacing the catalytic converter, be sure to check your warranty first before you head down to the local Midas. Some manufacturers offer extended warranties (as much as 60,000 miles) on converters, so you might be entitled to a free (or cheap) one at the dealership. If not, you might (depending on where you live) have some options at the local muffler shop which will save you over OEM. Just be sure you’re compliant with local emissions standards.

Mass airflow sensors are usually a bolt-off, bolt-on replacement. Don’t spend $200 just to have someone turn a few nuts for you.

The Five Most Common Causes of a CEL [Lifehacker]


30 Responses to Dealing with the Dreaded CEL

  1. george says:

    just be carefull. these just tell you what the computer thinks is wrong in certain areas. it takes further tests and much better $$$$$$$$ testers to pinpoint the actual defect. just throwing parts at it might fix it or it could empty yer wallet quickly.

  2. ChrisW says:

    An inexpensive reader is a good investment. Mine cost $40 and has paid for itself several times over. Assuming you have a basic knowledge of car repair:
    After getting the codes use internet forums to discover what they MEAN and find what other owners had to replace. Even if the parts don’t fix the problem those parts were likely to need replacement soon. And they won’t need replacement if you do take it to a pro.

  3. Old Tool Guy says:

    I call BS. All a code reader can do is tell you one of the reasons the computer in the vehicle is outside the normal or accepted parameters. A code reader in the hands of a parts store hack is even more dangerous.

    With over 1200 possible OBDII codes, telling someone that there are 5 common codes is similar to saying that a person who is sweating heavily could be having a heart attack.

    If you don’t respect a professional technician enough to pay him for his time, then certainly feel free to stumble your way through buying the wrong parts and busting off an O2 sensor by using your vice grips instead of the proper tool, then be my guest. Very disappointed in this website for posting this crap, though.

    • ambush27 says:


    • Jason says:


      Step 1 : Parts store says p0174 and p0440 …
      Step 2 : 50$ “universal” 02 sensor and a new gas cap…
      Step 3 : Crawl under some discount jack stands…
      Step 4 : Remove o2 sensor and realize your universal o2 sensor isnt the same (IT HAS NO PLUG…NOW WHAT?!)…now you need the right o2 sensor for 80-200$ depending on make , or you cheap out again and get some butt connectors (not the salmon ones…google them)…
      Step 4a : finally get it all back together and slap your new gas cap on…
      Step 5 : Parts store sets up the scanner and you clear your code , cause legally they aren’t allowed to now.
      Step 6 : CEL comes back on because the o2 code and the evap code ends up just being reaction codes. You just blew 200$ (jack stands , sensor , gas money , time)
      Step 7 : Take it to a shop , they scan your codes , look at a schematic , and see that you have a leaky injector and a saturated evap canister…
      Step 8 : Enjoy your new o2 sensor…

      Sorry to rant but I have customers tell me to “just hook it up to the machine that tells you whats wrong” … a doctor doesnt look at bloodwork and just say this is the problem. He looks at the information presented to him , interprets it , then makes an educated and informed diagnosis.

      The best thing to do is find a local shop that has a great reputation.
      Pay the hour diag ( if its more than 100$ run). From there , you can choose to tackle it yourself , or have the shop do it.

      Some of them will put the diag fee towards the repair . Some wont. ymWILLv


  4. AK-John says:

    My VW switches on the CEL light for everything. Why can’t the manufacturer install a code display for the user? It’s ridiculous to take my car to the shop to reset the CEL light for a turn signal bulb! I bought a pc based vagcom program to access the error codes and reset them as I go. Yes, I take responsiblity for my own repairs. I appreciate a professional tech when I need one, but the CEL light holds many people hostage to high shop prices ($125/Hr) to perform simple maintenance. The local VW shop charges $175 per side to replace the headlight bulb. I don’t need a professional tech for this.

  5. ben says:

    Yeah if you work on your own OBDII car you should get a code reader. And guess what! There’s an app for that! I got a bluetooth to OBDII dongle from ebay for ~$25.00 and I can do diagnostics from my smartphone with a $5.99 app. Fixed the wife’s VW. Father in law was very impressed 🙂

  6. Kurt says:

    I would add vacuum leaks to the short list of common problems. If you do have to replace an 02 sensor, check to make sure it is not caked in dirt or oil, as the sensor works by generating a voltage based on the differential between inside and out, and there are small holes that must not be obstructed for it to work properly – if you have an oil leak above it, fix it first.

    Even OBDII is not magic, and only guides the mechanic into the area of trouble, so a fault display on the dash would probably not be very helpful – the OBDII tester displays information in a stream that adds detail to the trouble code, such as the O2 switching rate, exact TPS value and other info that, if you know what you are doing, can be very helpful. It still pays to have a sound background in how the vehicle works however.

    As a retired mechanic, one information source that would have been very helpful to me is the Internet itself. With all the forums out there and a bit of knowledge, I have been able to quickly diagnose problems that would have taken a lot longer back in my wrenching days. A buddy had a Toyota with a bad ignition noise suppression capacitor (electronic ignition, so not an automatic replacement like the old days). Found the problem in minutes based on a bit of research. Would have been nice…

    BTW, we never called it the CEL back in the day, it was always the Check Engine Light. Heck, there were enough acronyms already LOL.

  7. Paul says:

    Uh, guys, most cars will have a way to read the code without a separate reader. And I can assure you at a reputable shop, unless it’s an obvious fix like a vacuum line visibly disconnected the first thing a mechanic will do is check the part, or system, listed on the code.
    An example, my wife’s ’04 Dodge Neon. I had to turn the key on, off, and on three times in less than five seconds to get a code readout on the odometer. P0456 and P0440 were both EVAP leaks. A hardened, cracked line and gas cap. P0340 was the CAM sensor. I replaced the plugs at the same time. Two hours, including cleanup, and less than sixty bucks and it’s all good again.

  8. Dave says:

    For my Mitsubishi to read the codes all I need is a paper clip or something similar to bridge the pins and set it into diagnostic modes, then read the flashes off the dash lights and look them up on the chart. Dead simple.

  9. Shopmonger says:

    Old Tool Guy: I respect all good technicians. But to make a blanket statement like that shows your own ignorance. The first thing any good tech is going to do is get he obdII codes. and the next step is to normally clear the code and make sure it was not simply a voltage spike or “outside given parameters” code. An aftermarket stereo with a good Sub and Amp can set off one of these if a Capacitor is not used.

    “With over 1200 possible OBDII codes, telling someone that there are 5 common codes is similar to saying that a person who is sweating heavily could be having a heart attack.” Yes but if a fat guy is sweating and it 50 degrees outside and he has not been running…then a heart attack is a reasonable explanation. Statistics exist for a reason. TM did not say that these are for sure the reason, but they are right in saying that these are the most common. I was a Tech and I was as you called them “Parts store hack” , and ASE certified parts store hack. And yes TM is correct.

    As for why cars don’t easily as quickly display the codes and reasons….1 is people panic, and the most important reason is why want people to go to the dealers ….DUH

    Im not saying there is some sort of conspiracy, they do want you to see a dealer when there is an issue…..


    • Old Tool Guy says:

      Your assertion that a technician will get and then clear the codes shows who is ignorant of proper repair procedures. I won’t enter into a pissing match with you, only tell you that I am a Master ASE tech with over twenty years experience in driveability. A voltage spike from a car stereo? please.

      You said it best: you “were” a tech. Don’t try to fool anybody into thinking that the fact you got hired into the job equates competence…especially since you obviously weren’t good enough to keep make a living at it. Yes, I believe you were a parts store hack if you believe the drivel you wrote.

  10. Jerry says:

    Looks like the readers split into two groups on this one – maybe even three. I like to believe that the folks on here know a little about mechanical things and wouldn’t very likely grasp the O2 sensor with vise grips but,…..
    For me? My Explorer popped up a check engine light. Local shop wants $80 to simply read the codes – no repairs, just read the codes. For $80 I bought a OBDII from – wait for it – Harbor Freight. Plugged it in and within a couple minutes had the possible causes. I started with the simplest – the gas cap. That fixed it. Just took the cap off and put it back on, cleared the codes and all was well. Just odd that it takes so long for the computer to realize there is a problem. It started when I filled up and the gas jockey failed to get the cap on right. However, the CEO didn’t come on until about 15 miles later.
    Anyway, I resolved the problem for the same price the local shop wanted to read the codes, AND I have a code reader now. Yes, it is HF but it works fine.

  11. John says:

    ChrisW had it right. Step one read the code. Step two google the code + the car model. Chances are dozens of people have already had the same problem and posted a fix. More than half the time this method gets me exact step by step instructions for correctly fixing the problem. If you’re the sort to just hand your car over to the dealer when the light comes on then I don’t know why you’re reading this website; turn in your wrenches 🙂

    • Ambush27 says:

      Just to be clear, I’m not against fixing your own car, but replacing parts based on someone elses diagnosis and recommendation is a good way to waste money. Sure There are certain problems that are common in one particular car line, but if your willing to buy a code reader you might as well get one with data, buy a dvom and a subscription to all data diy. If you can follow instructions you should be able to get through most p-codes. Which are generally the only ones that are going to set a malfunction indicator lamp(MIL).

      • John says:

        I’ve had access to factory service information for cars I’ve worked on. The vast majority of the time it’s not worth it. If your car is old enough to be out of warranty the best source of information about common problems is going to be internet forums devoted to that type of car.

  12. Toolaremia says:

    I will not skip the locking gas cap. With gas predicted to push $5/gallon this Summer and a 35 gallon tank in my truck, I’m not going to leave $175 unlocked.

  13. VW Nerd says:

    If you have a VW or Audi, then the go to company for a scan tool is Ross Tech, they make VAG COM. It works with all control modules that the dealer scan tool does, but cost less than $400 where as the the factory scan tool is about $10,000. Now if they would make a BMW tool, I’d be set for life.

  14. Shopmonger says:

    Actually my response ……….was from and ASE instructor. I asked him about this before I responded…as for the hiring of me……you can have your own opinion. I don’t care…..
    And yes voltage spikes create also code because there are some systems not isolated….

    Shop monger.

  15. Shopmonger says:

    Oh by the way old tool guy……….I too. Taught ASE courses so maybe you were one of my students….be mindful of what you say Mr master tech…..

  16. Dave says:

    A common code that will suck down the $$$ is the dreaded P0420, catalyst efficiency low.

    Knee-jerk reaction is to replace the cat and/or O2 sensors, but it is quite often a vacuum leak or an exhaust leak causing the code.

    There may be an ODB-II equipped car out there that gives you a way to read the code without a reader, but I have never heard of it. Paperclips worked with ODB-I.

    The most valuable advice I can give is to get the code then find a forum for your make&model, problems tend to be common.

  17. ShopMonger says:

    Dave Soooooo true, or even and exhaust leak….. Some time it trips the 02 sensor when the leak is above the o2 bung.

  18. russ says:

    I have to agree with some others here, a code reader, internet connection with access to forums, and common sense is a start to get your car problem resolved correctly. Depending on the code and symptoms I usually clear it first just to see if will come back. Then I hit the inet. But don’t take every answer you see on the inet as the holy grail answer.

    Another tool that I have just recently purchased is a data logger. At the present I am running it and getting a baseline with different parameters, sensors, speed, rpm, torque,timing, etc. It will also tell you the codes and allow you to clear them. The one I have is a dongle type. I pull it out connect it via a usb cable to my laptop and I have the data. It’s another tool. It’s in a testing phase so I can’t say it is as good as slice bread but I do like some preliminaries.

    Don’t think the mechanics will get it right (the first or second time) especially with the PO300 engine misfire codes. That bill can add up quickly, depending which route they take. Even if you don’t work on your car it is good to have a scan tool just to have that knowledge when you take your car in for repair.

    Coming up next is OBD-III. That’s another story. I believe that comes in 2016 but don’t quote me on that.

  19. TL says:

    Old Tool Guy needs to take a chill pill.

    A $20 code reader with an internet connection to tell you what the code means won’t solve your problem, but it can give you a direction. Where to go from there will depend on your skills, funding, and the code. What I’m willing to try fixing on a $50,000 Porsche is rather different from what I’ll try on a 12 year old $1200 Corolla. There is no reason for me to give a shop 25% of a car’s value to fix a P0171 code when a $3 can of sensor cleaner will often do the trick.

  20. Tony says:

    I’m with TL, Old Tool Guy needs to chill out.

    Often the fault is a loose or corroded connection, so check the wiring first. This is especially so when you’re getting multiple codes – check the wiring diagram to see if they’re all linked together on the harness.

    FWIW, doctors only use three codes for diagnosis: head, heart & cancer. Anything else is generally trivial. Old Tool Guy should listen to the chatter when when he picks up his pills, it’s really not what it’s like on TV. 5P (or PPPPP) is my favourite diagnosis.

  21. Shopmonger says:

    Russ, You are so right, even the mechanics get it wrong, because it is still interpreting what the computer information gives, use. An inexpensive code reader, and the power of google….because you can then find a hundred other folks who have the same car and the same code issues, and many times they have written up what the actual outcome was….. I always suggest googling (your car ) owners forum… there is a users and owners gorup for everything… especially cars..


  22. Old Tool Guy says:

    I understand all of you who think I need to be more relaxed about this.

    And as soon as you walk a mile in my shoes, having come on a website that says anybody with an internet purchased scan tool and enough free time can take food out of your children’s mouths, demean your career, and says you’re a crook for investing over $40,000 in tools and untold hours of unpaid time in continuing educational session but you charge too much for what you do, I promise to be very calm and tell you to chill out also.

    Feel free to spend all the time you want trying to fix it yourself and take the advice of internet experts who have never seen the car you’re working on. As a matter of fact, don’t bother trying to put it back together. I’ll just keep charging you for time and materials until it’s fixed right. Good luck.

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