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
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
- Find Your Engine’s Firing Order
Learn the firing order of the engine to identify cylinder three, which is the third cylinder to fire
- Remove the Ignition Coil
Remove the mounting bolt from the ignition coil, disconnect the electrical connector, and remove the ignition coil
- 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
- Remove the Spark Plug and Check it for Black/White Deposits
Check the spark plug for black soot, white deposits, or gloss
- 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
- 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
Learn How to Diagnose and Replace Parts Yourself
1A Auto has thousands of how-to videos with instructions from professional mechanics to help you diagnose and replace different parts on many makes and models.
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- How to Test If an Ignition Coil Is Bad