P0304 serious

Cylinder 4 Misfire Detected

What does P0304 mean?

The P0304 code means the vehicle's onboard diagnostic system has detected: Cylinder 4 Misfire Detected. This is a serious severity code.

Common Symptoms

  • Rough idle
  • Vibration at idle
  • Check Engine Light on or flashing
  • Reduced power

Probable Causes (Ranked by Likelihood)

  • Faulty spark plug on cylinder 4 Very High
  • Failed ignition coil on cylinder 4 High
  • Clogged fuel injector on cylinder 4 High
  • Low compression on cylinder 4 Moderate

Step-by-Step Diagnostic Procedure

  1. Swap spark plug from cylinder 4 with a known-good plug. If the misfire follows, replace all plugs.

  2. Swap coil pack from cylinder 4 to another position. If misfire code changes numbers, replace the coil.

  3. Test injector on cylinder 4 resistance and balance.

  4. Compression test cylinder 4 as a final step.

Frequently Asked Questions

Does P0304 mean my engine is damaged?

Not necessarily. Most P0304 codes are caused by a simple spark plug or coil failure. Extended misfiring, especially with a flashing CEL, can damage the catalytic converter. Diagnose and repair promptly.

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