DIY – Help! My “Check engine” light is on!


October 26, 2012 by quirkyuncle

Seeing a “Check Engine” light (CEL) on your dashboard is never a good thing. Knowing how to figure out what it means for yourself can save you a lot of stress and money.

Note: If your check engine light is flashing, this is an indication of serious problems. You should pull over and turn the car off as soon as it is possible (and safe), and find our what is wrong. Continuing to operate a car with the check engine light flashing can result in a catastrophic (as in very costly) engine failure.

Tools: Several free methods or optional purchase of a scan tool (see text)
Time: 5 minutes (to read the code)
Difficulty: Easy

Check engine lights come in several variants, depending on the make and age of the car you drive. In addition to the “Service Engine Soon” light shown above, some other light types include:

So, my light is on (not flashing). What do I do?
To remedy most causes of a CEL, you will need to follow the four (hopefully not five) R’s of auto repair:

  • Read the code
  • Research what the code means
  • Repair the problem
  • Reset the code
  • Repeat (if necessary)

Reading the code
Now that you know a code is in there, you can find out what it is in one of the following ways:

  • Read the report – If you failed your state emissions inspection, it will be on the summary sheet they give you.
  • Have someone read it for you – Many auto parts stores now read OBD codes as a free service. In some cases, they will even tell you the part you need and offer to sell it to you – if it’s super simple, they might even install it for free. Your mechanic or car dealer might be willing to read the code for you, but some charge for the service. (I’ve seen prices ranging from $20 to $150, so always ask first.)
  • Read it yourself – there are two ways to read the code yourself:
    • Ask the car – As silly as it sounds, some cars have an OBD interface built in that can be accessed by the owner. Search online for information about your particular vehicle. I had one car where you pressed the heater controls in a crazy sequence and the OBD code would then display on the heater temperature display. For another car, if you jumpered (shorted) two pins on the OBD connector with a paper clip, the check engine light would flash the ODB code. You never know until you check!
    • Use a code reader – The easiest way to read an OBD code is with a code reader, also known as a scan tool. You can find scan tools at local stores that sell tools or auto parts, or they can be purchased online. Scan tools are priced over a wide range, but I’ve seen them for less than $50 (very basic). Make sure you get one that allows you to clear codes after the repair, so you can make sure that you’ve fixed the problem. Also be sure to get a tool that is compatible with your car.

      Starting in 1996, all new cars sold in the USA were required to follow the OBD2 diagnostic interface. Some earlier vehicles used a variety of different interfaces, before the OBD2 standard was adopted. While all car manufacturers use the OBD2 standard as a baseline, most of them also generate additional diagnostic codes that are manufacturer specific. An OBD2 scan tool can generally read these codes, but often can’t tell you what they mean – not much of a problem, as you can typically search online for manufacturer code information.

      Dedicated scan tools have been around for years. A new twist is scan tools that work with applications that run in your smart phone, where a transmitter plugs into the car and sends the OBD data to your phone for display. Several companies market this type of product. Details can be found in the application store for your smart phone type.

      You can see a list of OBD2 readers that I’ve tested at OBD2 reader reviews, each review has links for ordering one.

      Reading the OBD2 code with a scan tool
      If you have a scan tool, this is how you read a code.
      Note – This procedure uses the BlueDriver Scan Tool from Lemur Monitors. The process for using other scan tools is similar.

      You can order a BlueDriver Scan Tool from

      1. Locate the OBD2 connector in your car. In most newer cars, it is under the dashboard on the driver’s side. In some cars the connector is behind a trim panel. See your owners manual or look online for details. All OBD2 connectors look like the one shown below.
      2. With the car turned off, align the scan tool connector with the OBD2 connector on your car; then, plug it in.

        In this case, the LED on the BlueDriver Scan Tool transmitter lights, indicating that it is properly connected and receiving power.
      3. If you are using a smart phone scan tool, turn on bluetooth and establish a connection with the bluetooth transmitter for the scan tool.
      4. Turn the car on; then, turn on the scan tool or open its application. If you are using a BlueDriver Scan Tool, the blue LED on the transmitter flashes to indicate that a connection has been established.
      5. Select your scan tool option. Since we want to read our diagnostic codes, we’d select Scan for Trouble Codes.
      6. After the codes are downloaded, the results are displayed on the scan tool.
      7. In some scan tools, you can click on a code to get additional details.

    Now that you’ve determined what your trouble codes are, it’s time to see what they mean.

Researching what the code means
Diagnostic codes are cryptic and confusing, and their short descriptions are not much better. While the codes themselves are rather generic, their causes are very specific and vary by vehicle. An experienced mechanic can often recognize the causes quickly, but an amateur needs help. You can get information about trouble codes and their meaning in one of the following ways:

  • Ask your local auto parts store – Parts stores have an extensive database of information that is tied to the diagnostic codes for your car. Most are happy to look up a code and offer to sell you the parts that you need to fix it, or direct you to a place where you can obtain parts that they do not carry. If the parts store was the place that scanned your code, they will likely give you a list as part of their report – if they don’t, ask, even if they don’t have the part you need.
  • Search online – Tons of information about diagnostic codes can be found online – just do a search for the code along with your vehicle type (for example, p0455 2000 mustang). Information about all of the OBD2 codes is at There are also many online forums for general auto repair and for your specific car type that have lots of useful information, including specific diagnosis and detailed repair procedures (often with pictures). Forum members are knowledgeable and willing to help – just use good forum etiquette and do a search before asking a question to make sure the issue was not already addressed.
  • Look in your repair manual – If you have a repair/maintenance manual for your car, it will fully explain all the diagnostic codes and provide solutions specific for your vehicle.
  • Consult a dedicated smart phone application – There are OBD2 code database applications available for most smart phones that can give you fairly comprehensive data. Some are free, some are not. Some are good, some are not. Read the reviews and try before you buy, if possible.

    The BlueDriver Scan Tool gives you option of getting more detailed information for a specific trouble code by entering your vehicle identification (VI) number. This is an in-application purchase option that gives you five detailed reports for a small fee. A sample of what a detailed reoprt contains is shown below.

Always consider symptoms and conditions when reviewing your diagnostic codes – what were you doing when the light first came on? Did you just get gas? Did the car start running differently around the time the light came on? Observations are a huge factor in diagnosing problems, even of you bring your car to a mechanic for repair. Mechanics charge by the hour, so any information you can give them when you drop your car off might help save time and reduce your bill.

Repairing the problem
Performing the actual repair can be the tricky part. Some repairs are quick and simple, while others require significant time, skills, and special tools. Often the diagnosis is not clear and you’ll need to decide which of several repair parts needs to be replaced.

Having experience makes the diagnosis easier, but even the best mechanic does not replace the correct part on the first try every time. You’ll never learn unless you try! If the parts are affordable and you have the ability to install them yourself, take the opportunity to give it a shot and learn from the experience. If the parts are expensive and you are unsure of either your diagnosis or abilities, consider getting some help.

Modern cars are very complicated, but often the parts most prone to failure are those that are simple and cheap. The engine compartment is a harsh environment and often a “failed” part does not actually need to be replaced and can be returned to operation merely by cleaning or un-sticking and lubricating it. Repairs like this cost little, aside from your time.

For example, the P0455 code, shown above, was caused by a loose gasoline filler cap – we made sure it was tightened all the way and we were done! Imagine paying someone $150 to diagnose and fix that!

Resetting the code
Clearing the diagnostic code allows you to quickly see if your repair was successful. Some scan tools have this capability. If you are purchasing a scan tool, I’d advise you to spend a little extra money for one that includes the reset/clear function.

If you are not able to clear your diagnostic code with a scan tool, you’ll need to drive your car a while to see if the code clears itself. Automotive computers are pretty smart and adaptable. They will notice when a failure has been corrected, but some wait a while before turning off the check engine light, just to be sure. I’ve owned some cars that turn the light off immediately, and others that I’ve needed to drive for a month (30 hours of run time) before the light would turn off. You can check online to see how your particular vehicle computer automatically responds to repairs.

Note – Clearing the codes won’t work for cheating your way through an emissions test, because the testing center can tell that you’ve reset the computer. They will make you drive the car a certain distance (60 miles, where I live) after performing an OBD reset to make sure that no trouble codes reappear.

Repeating the process (if necessary)
Auto repair is not an exact science. Vehicles, especially as they get older, can experience multiple simultaneous failures. Don’t be discouraged. When codes and diagnosis are unclear, expect that you might not hit the target on the first shot.

Bear in mind that if your vehicle is older, the parts you are replacing were likely reaching the limit of their useful life, anyway, so your car will likely run better afterward even if it is not fully fixed. Save your old parts, if a replacement did not solve the problem – you can always put them back later, if the replacement part ever fails.

Good luck!

Did you find this posting useful? Did it save you time or money? If so, consider making a donation.


  • 28 September 2017 – Added info and link to OBD reader reviews.


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