Misfire Codes After A Spark Plug Replacement (Video)
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Misfire Codes After A Spark Plug Replacement (Video)

After replacing spark plugs, you may get a handful of misfire codes. This video is sponsored by Autolite.

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You just replaced the spark plugs, and the hood is closed. What should you do next?

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Yes, a test drive is required, but you might want to do a “post-scan” of the vehicle for quality control before you do that.

There are two types of codes set by the ignition system. First, these are hard codes that typically cover the electrical operation of the ignition system. These codes are set by the power, ground and other circuits going to the coil. Some codes indicate a circuit is open or has a resistive value outside of parameters. For instance, code P0351 indicates “Ignition Coil “A” Primary/Secondary Circuit Malfunction.” It could mean that the ignition coil number one is faulty. But more than likely, it means that the connector was not fully seated after the spark plug was replaced.

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The second type of codes are generated by monitors that run continuously over a set number of crankshaft rotations. In the case of spark plugs, this monitor is the “misfire monitor.” The misfire monitor uses the crankshaft position sensor to observe the movement of the crankshaft. The monitor counts the individual misfires by a cylinder that has exceeded the values. The monitor looks at a defined number of crankshaft rotations to determine if a misfire is a misfire and not a bumpy road or torque converter lock-up event.

You might get a misfire code after spark plug replacement for an individual cylinder like a P0303. The code indicates a misfire in cylinder three. Sometimes the spark plug is dropped, and the ground electrode and the center electrode can make contact. Or the problem could be with the connection to the coil. Should you clear a P0351 or misfire code like P0303 using a scan tool? No, codes like these use continuous monitors. These types of codes will clear automatically from active status if the problem is resolved or the monitor is completed without incident. But it might take between one to three key cycles to turn off the check engine light. Yes, you can erase the codes, but you will not be able to confirm that you resolved the issue.

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This video is sponsored by Autolite

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