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Diagnosing a Cylinder Misfire

Have you ever had your car suddenly start to sputter on acceleration? These hesitations can be from a variety of things that are starting to malfunction, including the ECU, fuel pump, fuel injectors, cam or crank sensors, 02 sensors, coils, or spark plugs. More times than not, however, it seems that either of the last two are common failing points on cars and give these symptoms. Fortunately, they’re pretty easy to diagnose if you have access to the coils.

If you’ve had enough sputtering to trigger a check engine light, an automotive diagnostic tool or scanner that plugs into the OBD2 port (for cars 1996+) can save you some trouble and tell you exactly which cylinder poses the problem. If you don’t have a scanner, you’ll just have to go old school.

In most modern-day vehicles, engines have a plastic cover, which will usually expose a coil-on-plug setup upon removal. If your car is idling funny and you can gain access to the coils, you can easily figure out if your sputtering issue is coming from the ignition system or elsewhere.


Test #1  If the car idles like it’s running on one or two less cylinders, have the car running. As you unplug coil #1, listen to the tone of the engine—does it change? If unplugging the coil makes the car suddenly run worse, then you know things are normal in that cylinder. Plug the coil back as it was, and move onto the next cylinder.

Unplugging an Ignition Coil

Now, let’s say you get to cylinder #2 and you unplug the coil and it doesn’t change a thing—then you know that the coil or the plug is a potential culprit here. We’ve at least narrowed something down to this cylinder. To find the specific problem, swap coils from cylinders #1 and #2 and retest. If nothing changed, and disconnecting cylinder #2 is still not making any change to the sound of the engine, then you know it’s not the coil, but quite possibly the spark plug of cylinder #2.


Test #2  We’ll now move on to check the spark plug. What you can do here is either put in a new plug, or simply swap plugs with cylinder #1 if you don’t have a new replacement yet. If the problem moved from cylinder #2 to cylinder #1, this confirms you have a faulty plug.

Checking the Spark Plug 

Check the condition of each plug when you pull them out. Check the color—you’ll want a tan or light grey hue, which typically indicates proper combustion. If the plug is wet or very black in color, then you could be running too rich. If pieces of the plug are missing, there are other issues with the car that should be checked by a professional mechanic immediately.

Reading a Spark Plug with Normal Wear

Test #3  If by checking all cylinders you know that the plugs and coils are okay, then see if you can do the same with the fuel injectors—again, one by one. However, you won’t have to remove them, just unplug the connector and see if the idle changes. If it doesn’t, then you may have a faulty fuel injector.

If neither of these tests pinpoint a culprit, well then you’ve at least figured out that, while there is something going on with the car, it’s not the coils, plugs, or fuel injectors.