Overview

A misfire occurs when one or more cylinders fail to contribute normally to engine power.

Common Symptoms

Common symptoms of an engine misfire include:

  • Rough idle
  • Hesitation
  • Loss of power
  • Flashing check engine light
  • Misfire-related diagnostic trouble codes

Possible Causes

Possible causes of an engine misfire include:

  • Worn or fouled spark plugs
  • Failed ignition coil
  • Fuel injector problem
  • Vacuum or intake air leak
  • Low cylinder compression
  • Incorrect valve timing
  • Engine control system fault

Diagnostic Approach

Identify the affected cylinder and determine whether the cause is related to ignition, fuel, air, mechanical condition, or engine control.

  • Check stored and pending diagnostic trouble codes
  • Review freeze-frame data
  • Monitor misfire counters when available
  • Inspect ignition components
  • Evaluate fuel delivery and injector operation
  • Check for intake or vacuum leaks
  • Perform mechanical testing when necessary