P0303 serious

Cylinder 3 Misfire Detected

What does P0303 mean?

The P0303 code means the vehicle's onboard diagnostic system has detected: Cylinder 3 Misfire Detected. This is a serious severity code.

Common Symptoms

  • Rough idle
  • Flashing CEL under load
  • Power loss
  • Fuel odor from exhaust

Probable Causes (Ranked by Likelihood)

  • Faulty spark plug on cylinder 3 Very High
  • Failed ignition coil on cylinder 3 High
  • Clogged fuel injector on cylinder 3 High
  • Low compression on cylinder 3 Moderate

Step-by-Step Diagnostic Procedure

  1. Swap the cylinder 3 coil to an adjacent known-good position. If the misfire follows the coil, replace it.

  2. Inspect and replace the spark plug on cylinder 3.

  3. Perform injector balance test. Low contribution on cylinder 3 indicates an injector fault.

  4. Compression test cylinder 3 if other fixes do not resolve the code.

Frequently Asked Questions

Is cylinder 3 the same location on every engine?

No. Cylinder numbering varies by engine configuration and manufacturer. On V6 engines especially, cylinder 3 may be on either bank. Always consult your vehicle service manual.

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