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P0303 Fault Code: Causes, Symptoms, and Fixes


If you’ve ever experienced a check engine light (CEL) with a P0303 code, you may be wondering what it means and what you should do about it.

P0303 indicates a misfire on cylinder three.

I’m John Cunningham, a qualified mechanic, you are in the right place, and very shortly, you’ll have a good understanding of the cause, the diagnosis process, and the fix.

We’ll cover the following:

What is P0303?

Engine cylinder layout

The code is triggered by the engine control module (ECM) when it detects a deviation from the expected combustion event in that particular cylinder.

P0303 Symptoms

Here are some of the most common symptoms associated with a P0303 code:

  • Check engine light: The CEL will illuminate on your dashboard.
  • Rough idling: You may notice that your engine is shaking or vibrating more than usual while idling.
  • Loss of power: Your vehicle may struggle to accelerate or may feel like it’s not running as smoothly as it should.
  • Engine misfiring: You may hear or feel a “hiccup” in the engine’s power delivery.
  • Poor fuel economy: If the misfire is severe, your vehicle’s fuel efficiency may suffer.
  • Failed emissions test: A misfire can cause your vehicle to fail emissions testing.
  • Smell of unburned fuel: If the misfire is severe enough, you may smell unburned fuel in your exhaust.

What causes P0303?

P0303 can be caused by a variety of issues, including:

  • Faulty ignition system components: Spark plugs, ignition coils, and spark plug wires can all contribute to misfires if they’re faulty or worn out.
  • Fuel delivery issues: A clogged fuel injector or fuel filter can cause a lean or rich fuel mixture, leading to misfires.
  • Mechanical problems: Low compression in the third cylinder, a damaged valve or piston, or a worn camshaft can all cause misfires.
  • Vacuum leaks: Leaks in the engine’s vacuum system can cause misfires by affecting the air/fuel mixture.

How to diagnose P0303?

To diagnose a P0303 code, a mechanic will typically perform the following steps:

  • Use a diagnostic scan tool to read the DTC and any related freeze frame data.
  • Inspect the ignition system components for wear, damage, or corrosion.
  • Check fuel system components for proper operation and pressure.
  • Perform a compression test to check for mechanical issues.
  • Inspect the vacuum system for leaks or damaged hoses.

Common fix for P0303

Once the underlying cause of the P0303 code has been diagnosed, the fix will depend on the specific issue. Here are some common fixes for P0303:

  • Replace faulty ignition system components: If the issue is with spark plugs, ignition coils, or spark plug wires, they may need to be replaced.
  • Clean or replace fuel system components: If the fuel injector or fuel filter is clogged, cleaning or replacement may be necessary.
  • Repair mechanical issues: If the issue is with the engine’s mechanical components, such as low compression or a damaged valve, they may need to be repaired or replaced.
  • Fix vacuum leaks: If there are leaks in the engine’s vacuum system, they will need to be repaired or replaced.

Sum Up

P0303 code can be a sign of several underlying issues. By paying attention to the symptoms and taking your vehicle to a trusted mechanic, you can diagnose and fix the issue before it causes further damage.

About the Author

This article was created with the assistance of AI technology to aid the author, John Cunningham, who is a seasoned Red Seal-certified auto technician with more than 25 years of experience in vehicle repairs. However, please note that John Cunningham has edited the content to ensure accuracy and quality.

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