Ignition

P0306 — Cylinder 6 Misfire Detected

Severity: medium Drive with care, to a shop

What P0306 means

The engine control unit noticed uneven crankshaft rotation that points to incomplete or missing combustion in cylinder 6. A cylinder needs three things to fire properly: a spark, the right amount of fuel and enough compression. If any of them fails, the cylinder misfires. Cylinder numbering depends on the engine layout, so confirm where cylinder 6 is on your model.

Symptoms

  • Check engine light on or flashing
  • Rough idle and engine shake
  • Hesitation and power loss when accelerating or under load
  • Worse fuel economy
  • Raw fuel smell from the exhaust
  • Engine running as if a cylinder is dead

Possible causes

  1. Failed ignition coil on cylinder 6
  2. Worn or fouled spark plug on cylinder 6
  3. Clogged or leaking fuel injector on cylinder 6
  4. Vacuum leak near the cylinder 6 intake runner (gasket, cracked hose)
  5. Low compression in the cylinder (valve, rings, head gasket)
  6. Damaged wiring or connector at the coil or injector
  7. Less commonly, a crankshaft or camshaft sensor problem

How to diagnose

  1. Read the codes and freeze frame data: load, rpm and temperature when the fault set.
  2. Check the per-cylinder misfire counters to confirm only cylinder 6 is affected.
  3. Inspect the cylinder 6 spark plug for wear, oil, deposits and gap.
  4. Swap the coil (and plug) with a neighbor cylinder, clear the code and see if the misfire follows.
  5. Test the injector: resistance, clicking and cylinder balance.
  6. Inspect the intake for vacuum leaks with a smoke machine or spray test.
  7. Check coil and injector connectors and wires for corrosion or breaks.
  8. If electrical checks pass, run a compression test and a leak-down test if needed.

Common fixes

  • Replace the cylinder 6 ignition coil
  • Replace the spark plugs, ideally as a set
  • Clean or replace the fuel injector
  • Fix the vacuum leak: replace the intake gasket or hose
  • Repair the engine if the cause is mechanical (valves, rings, head gasket)

Questions and answers

Can I drive with a P0306 code?

Only briefly and gently if the light is steady. If it flashes, stop driving, because the catalytic converter can overheat and fail.

What most often causes P0306?

Most often it is a worn spark plug or a failing ignition coil on cylinder 6. Swapping them with another cylinder is the quickest test.

How do I know it is not the coil?

If the misfire stays on cylinder 6 after swapping the coil and plug, check the injector, vacuum leaks and compression next.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.