Engine Misfire? How to Diagnose Trouble Code P0303

If you have an engine misfire, which will feel like a rough idle or rough running engine while driving, and a check engine light for trouble code P0303, it means there is an issue with cylinder 3 on your engine. This article explains what to do and how to fix this problem if you’re experiencing it.

How to Fix Code P0300

Symptoms of an Engine Misfire

  • Rough running engine that feels like it’s vibrating or shaking
  • Check engine light for code P0303
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How to Diagnose Code P0303

The ignition system may have a distributor with a cap and rotor, spark plug wires, or the ignition coils may be controlled electronically by the computer. If your vehicle has more modern technology (model year in the 2000s), it likely has a “coil-on-plug” ignition coil, meaning there is one electronically-controlled ignition coil per spark plug.  

These steps explain how to diagnose a coil-on-plug ignition coil.

Steps for Diagnosing Code P0300

  1. Find Your Engine’s Firing Order

    Learn the firing order of the engine to identify cylinder three, which is the third cylinder to fire

  2. Remove the Ignition Coil

    Remove the mounting bolt from the ignition coil, disconnect the electrical connector, and remove the ignition coil

  3. Inspect the Ignition Coil for Cracks or Black/White Deposits

    Visually check the coil for cracks in the plastic or at the boot, or traces of black soot or white coloring

  4. Remove the Spark Plug and Check it for Black/White Deposits

    Check the spark plug for black soot, white deposits, or gloss

  5. Test the Ignition System with an Advanced Scan Tool

    You can run and test the engine with an advanced scan tool that will monitor the system. If not, you can try another simple test

  6. Test the Ignition System by Relocating the Spark Plug and Coil

    Remove the ignition coil and spark plug from cylinder 3 and swap it with one from another cylinder. Then clear the codes, run the engine, and see if the misfire follows the location of the ignition coil and spark plug, indicating they are bad.

    If the misfire follows both, replace the spark plug and ignition coil

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Engine Misfire? How to Diagnose Trouble Code P0303 - 1A Auto
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Engine Misfire? How to Diagnose Trouble Code P0303 - 1A Auto
If your engine is misfiring and you have a check engine light for code P0303, this article and videos explains how to fix it
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