P0306 Code (Symptoms, Causes, and How to Fix)

Vehicles are powered by the combustion of air and fuel, which takes place in different engine cylinders. This is how a vehicle generates the power it needs to accelerate and spin its wheels.

A vehicle usually has 4 to 8 engine cylinders depending on its power needs. When one of these cylinders misfires, the car’s computer sends out a code specifying the faulty cylinder. Although the engine is still able to generate some power after one of its cylinders misfires, it is not enough to sustain the acceleration needs of the driver. 

What Does Code P0306 Mean?

OBD-II Trouble Code P0306 Description
Cylinder 6 Misfire Detected

Code P0306 occurs when the ECM (engine control module) detects a misfire on engine cylinder number 6.

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When cylinder number 6 misfires, it means that the air and fuel mixture in that cylinder has failed to ignite. As a result, the engine’s speed fluctuates, causing the crankshaft position sensor signal to vary. The ECM detects this and reports a misfire in the specific cylinder.

See Also: P0300, P0301, P0302, P0303, P0304, P0305, P0307, P0308, P0316

Symptoms of Code P0306 

Usually, there will be some noticeable signs that accompany a P0306 error code, and they include:

Causes of Code P0306 

Many things can cause the error code P0306, but these are the most common.

Faulty Spark Plug The bad spark plug may not properly fire in cylinder 6 due to insulation cracks or fouling.

Faulty Ignition Coil – Cracks in the insulation or open circuit in the ignition coil may cause it to stop firing cylinder number 6.

Worn or Damaged Spark Plug Wire – If a plug boot or spark plug is leaking ignition spark to the ground, cylinder number 6 may stop firing.

Leaky Valve Cover – If the valve cover is leaking oil into the spark plug holes, it may short the spark plug ignition for cylinder number 6. 

Low Compression – Low compression in the cylinder due to defective valves or rings may cause a misfire. 

Clogged, Dirty or Damaged Fuel Injector – Insufficient fuel in the cylinder can cause a misfire. A faulty fuel injector causes low pressure affecting the cylinders. 

Is Code P0306 Serious?

Driving with a misfiring cylinder is dangerous. A misfire in cylinder number 6 causes the engine to run excessively rich, damaging the catalyst. It also affects how your engine runs, makes it hesitate on acceleration, and uses more fuel.

DTC P0306 is not something to be ignored and should be repaired immediately. Ignoring this code can cause damage to your catalytic converter, which is quite costly to replace compared to repairing a misfiring cylinder.

See Also: P0335 Code (Crankshaft Position Sensor Malfunction)

How to Fix Code P0306

To fix this issue, you will need to check all the possible causes listed above. Start with these steps to work on fixing a P0306 error code.


Begin by inspecting the connectors and wiring harness for any damage. Use a good OBD2 scanner to record all error codes and freeze frame data. Use this information to do a quick visual inspection of the fuel injector, ignition coils, and related wiring. 

Look for any loose connections or damaged wiring. Try removing the ignition coil and swap it with another cylinder to check if that is the problem. Clear the engine and ETC codes, then do a road test to confirm if the problem comes back.

Conduct a visual check of the spark plug. Check for any signs of damage or fouling and replace where necessary. Swap the spark plug to another cylinder and see if the misfire follows. If the ignition system is not the problem, check for any vacuum leaks related to the number 6 cylinder.

If nothing is found visually, you may need to pull up the fuel trim parameter identifiers (PID) on a scan tool. You can also use an unlit propane torch or a smoke machine to check for any vacuum leaks. 

If there are no leaks, the next step is to check the fuel injector of cylinder number 6. If no issues are found thus far, there is likely a mechanical problem somewhere.

Conduct a compression and leak-down test to confirm. Look out for any broken valve spring, carboned valve, burnt valve, damaged piston, a worn camshaft lobe, or worn piston rings.

Other Causes of Error Code P0306

Use the results you found in the diagnostic tests to fix the issue. Check whether the cylinder’s spark plugs and ignition coil are working well or not. If they are working well, another problem near these components may be causing the misfires.

Cylinder number 6 may be having compression issues due to faulty rings and valves. Check if oil is leaking from the valve cover and flowing through the holes in the spark plugs. Whatever the problem is, have it repaired immediately. The longer you wait, the more permanent the damage will be.

Mark Stevens

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