AutoDiag

P0309

Cylinder 9 Misfire Detected

High· Fix promptlyIntermediatePowertrainGeneric, all makes

What it means

The ECU has detected that cylinder 9 is misfiring: the fuel in that cylinder is not burning properly. Because only one cylinder is affected, the cause is usually a part that belongs to that cylinder, such as its spark plug, ignition coil or fuel injector.

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 engine shaking
  • Loss of power
  • Jerking under acceleration
  • Poor fuel economy

Probable causes

Most likely first

  1. 1Worn or fouled spark plug in cylinder 9
  2. 2Failed ignition coil (or plug wire) on cylinder 9
  3. 3Clogged or failed fuel injector on cylinder 9
  4. 4Vacuum leak near that cylinder (intake gasket)
  5. 5Low compression: burnt valve, worn rings or head gasket leak
  6. 6Damaged wiring to the coil or injector

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. 2Swap the ignition coil from cylinder 9 with a neighbouring cylinder, clear codes and drive. If the misfire moves to the other cylinder, the coil is bad.
  3. 3Remove and inspect the spark plug. Swap it to another cylinder in the same way if it looks worn or cracked.
  4. 4If the coil and plug are fine, swap or test the injector (listen with a mechanic's stethoscope for clicking, or check its resistance).
  5. 5Do a compression test on cylinder 9. A reading more than about 10–15% below the others points to a mechanical problem.
  6. 6Look for oil in the spark plug well, which can short out the coil (leaking valve cover or tube seals).
  7. 7Check for technical service bulletins (TSBs) or software updates for your make and model; some codes have known factory fixes.

Common fixes

  • Replace the spark plug
  • Replace the ignition coil
  • Replace or clean the fuel injector
  • Repair a vacuum leak at the intake gasket
  • Repair the valve, rings or head gasket if compression is low

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.