AutoDiag

P0300

Random/Multiple Cylinder Misfire Detected

High· Fix promptlyIntermediatePowertrainGeneric, all makes

What it means

The ECU has detected misfires in more than one cylinder, or misfires that move around between cylinders. A misfire is when the fuel in a cylinder does not burn properly. Because several cylinders are affected, the cause is usually something they all share, such as fuel pressure, a vacuum leak or worn spark plugs across the board.

Is it safe to drive?

Avoid driving if the check engine light is flashing. A flashing light means a severe misfire that is dumping unburned fuel into the exhaust, which can overheat and destroy the catalytic converter within minutes. If the light is steady, drive gently to a repair shop.

Symptoms

  • Check engine light on or flashing
  • Rough idle and shaking
  • Hesitation, jerking or loss of power when accelerating
  • Poor fuel economy
  • Fuel smell from the exhaust

Probable causes

Most likely first

  1. 1Worn or wrongly gapped spark plugs
  2. 2Vacuum leak (cracked hose, intake gasket, PCV valve)
  3. 3Low fuel pressure (weak fuel pump, clogged fuel filter)
  4. 4Dirty MAF sensor causing a lean mixture
  5. 5Failing ignition coils or plug wires
  6. 6Bad or contaminated fuel
  7. 7EGR valve stuck open
  8. 8Mechanical problem: low compression, jumped timing chain or belt

How to diagnose it

Work through these in order before buying parts

  1. 1Record the code and its freeze frame data (speed, load, temperature when it set), then clear it and see if it comes back.
  2. 2Look at the misfire counters on live data (Mode 6) to see which cylinders are misfiring and how often.
  3. 3Check for other codes, especially lean codes (P0171/P0174) and cylinder-specific misfire codes. Fix those first.
  4. 4Check fuel trims. High positive trims point to a vacuum leak or low fuel delivery.
  5. 5Inspect the spark plugs: worn electrodes, oil fouling or carbon show different problems.
  6. 6Listen and smoke-test for vacuum leaks.
  7. 7Test fuel pressure against specification.
  8. 8If the basics check out, do a compression or leak-down test.
  9. 9Check for technical service bulletins (TSBs) or software updates for your make and model; some codes have known factory fixes.

Common fixes

  • Replace spark plugs (and coils or wires if worn)
  • Repair vacuum leaks
  • Clean the MAF sensor
  • Repair fuel delivery
  • Repair mechanical faults found in testing

Guides

Diagnostics

How to read a check engine code (and what to do next)

A plain-English walkthrough of plugging in an OBD-II scanner, reading the code, and deciding whether it is safe to keep driving.

This information is general guidance for all makes. Exact causes, specifications and procedures vary by vehicle, so confirm with your service manual or a qualified mechanic before replacing parts.