P0306 Code: Cylinder 6 Misfire Detected

What the P0306 code means

The misfire detection system identifies the affected cylinder by measuring the instant loss of crankshaft speed at the exact moment each combustion event should happen. The P0306 code corresponds to cylinder six.

On a V6, cylinder six usually sits at one end of the rear bank. On many engines that is the most compromised spot: less ventilation, more heat and the worst access.

That combination explains two things that repeat with this code. The first is that the misfires show up sooner when hot and under load, when the coil has to produce more voltage and the degraded insulation stops holding up. The second is that many vehicles set the same code every few years. It is not bad luck or a defective part, it is the temperature of that position, and it is better treated as a heat problem than as a component failure.

There is one check that saves a lot of time on this cylinder: see whether the misfire goes away when you switch off the air conditioning or drive without load. A cylinder that only misfires when the engine is working hard almost always has a spark problem, because under load the coil has to produce more voltage to jump the gap in a chamber at higher pressure.

There is also a number worth writing down before you start: how many misfires this cylinder has racked up compared with the others on the same bank. If all three cylinders on the rear bank are accumulating misfires even though only one has set its code, stop looking for an individual component. In that case the cause is usually shared: insufficient fuel pressure that hits the bank farthest from the pump first, a leak in the common part of the manifold or a ground problem in the ignition system on that side.

P0306 code symptoms

• Check engine light flashing during the misfire.
• Clear vibration at idle.
• Loss of power, more obvious on hills and when passing.
• Jerking when you accelerate with a hot engine.
• High fuel consumption and a smell of unburned gas.
• Uneven starting.
• The misfire comes back periodically on the same cylinder even though the part was replaced.
• The misfire goes away when you switch off the air conditioning or drive without load.
• Other cylinders on the same bank are racking up misfires even though their codes have not set.
• Occasional black smoke when the engine is pushed on a climb.

Likely causes of the P0306 code

1. Cylinder 6 coil degraded by constant heat exposure.
2. Cracked wiring insulation in the hottest area of the engine, with shorts only when hot.
3. Worn spark plug or one with the wrong heat range.
4. Clogged injector or one whose connector has an intermittent contact.
5. Vacuum leak in the runner for that cylinder.
6. Low compression from mechanical wear or a valve that does not seat.
7. Low-quality replacement coil that cannot handle the temperature in that position.
8. Deteriorated head gasket on the rear bank, in the more serious cases.
9. Corroded ground for the rear bank ignition system, which affects several cylinders at once even though only one sets the code.
10. Insufficient fuel pressure, which hits the bank farthest from the pump first.
11. Spark plug tube with collected oil from a leaking valve cover gasket on the rear bank.

How to diagnose the P0306 code step by step

1. Check whether the code has appeared before on the same cylinder. A repeat misfire points to an underlying heat problem and not the part that was replaced.
2. Examine the wiring insulation in that area. If it is hardened or brittle, the short only happens when hot.
3. Swap the coil with another cylinder and check whether the misfire moves.
4. Watch the misfire counter while you accelerate. A weak coil mostly fails under load.
5. Check the look of the spark plug and its gap.
6. Check the engine operating temperature, because a weak cooling system punishes this position first.
7. Look for vacuum leaks in the intake runner for that cylinder.
8. Measure compression as the last step if nothing above explains the misfire.
9. Check whether the misfire goes away without load. A cylinder that only misfires when the engine is pushed points to spark before fuel or mechanical causes.
10. Compare the accumulated misfires of the three cylinders on the bank. If they all add up, look for a shared cause and not an individual component.
11. Remove the coil and look inside the spark plug tube. Collected oil sends the spark to ground, and you cannot see it without this step.
12. Measure the voltage drop across the ground of the ignition system for that bank with the engine running.

How to fix the P0306 code

• Replace the coil with one from a recognized manufacturer. Cheap ones do not handle the temperature in that position.
• Protect the harness with a heat sleeve when you repair it and route it as far as you can from the exhaust manifold.
• Change the full set of spark plugs with the manufacturer part number and heat range.
• Clean or replace the affected injector and secure its connector.
• Repair the vacuum leaks found at that runner.
• Check the cooling system if the code keeps returning.
• Take on the mechanical repair if compression confirms it.
• Clean and tighten the ground for the rear bank ignition system down to bare metal.
• Replace the valve cover gasket and clean the spark plug tubes if oil has collected, before you install new spark plugs.
• Check and correct the fuel pressure if all three cylinders on the bank are accumulating misfires.

Technical reference data

• Expected compression: 10 to 14 bar (145 to 200 psi), with a maximum difference of 10 percent between cylinders.
• Ignition coil primary resistance: 0.5 to 2 ohms, always measured cold.
• Ambient temperature next to the exhaust manifold: it can exceed 150 degrees Celsius (302 degrees F) while the engine runs.
• This code only exists on engines with six cylinders or more.
• Brittle insulation indicates long-term heat exposure, and the whole section should be replaced.
• Misfire only under load: a spark problem. Misfire across the whole range: fuel or mechanical.
• Allowable voltage drop across the ignition system ground: below 0.2 volts.
• Several cylinders on the same bank with accumulated misfires point to a shared cause, not an individual component.

Brand-specific notes

If this code has returned more than once on the same vehicle, stop treating it as a bad part and treat it as a heat problem: harness protected with a sleeve, a quality coil and a cooling system in good shape.

Whenever the intake manifold comes off, take the chance to check the coils and spark plugs on the other cylinders of the rear bank too. They are under the same conditions and tend to fail shortly afterward.

Frequently asked questions about P0306