P0306 – Cylinder 6 Misfire Detected

The trouble code P0306 indicates that cylinder number 6 is misfiring. Along with a check engine light, you may notice that your vehicle runs rough or has trouble idling. A P0306 is something that should be repaired right away. Ignoring misfire codes can cause damage to your catalytic converters, and that is generally a very expensive repair.

The P0306 code might mean that cylinder 6 is misfiring
The trouble code P0306 indicates that cylinder number 6 is misfiring

P0306 Definition

P0306 (Generic): Cylinder 6 Misfire Detected

What Does P0306 Mean?

Your vehicle’s engine has four cycles, intake, compression, combustion, and exhaust. Air and fuel enter the combustion chamber through the intake valve during the intake stroke. The air/fuel mixture is then compressed by the upward movement of the piston while the intake and exhaust valves are closed. Then spark is introduced during the combustion cycle which ignites and forces the piston back downwards. Then the exhaust valve opens and the piston will force the exhaust gases out into the car’s exhaust system.

When the Engine Control Module detects the piston in the number 6 cylinder is not being forced down during the combustion cycle it will cause a P0306 trouble code. This can be caused by a variety of different reasons, both mechanical and electrical. Be sure to check your service manual to identify the correct cylinder before getting started.

What Are The Symptoms Of The P0306 Code?

In most cases, there will be some very noticeable symptoms that come with a P0306 trouble code.

  • Illuminated Check Engine Light
  • Rough idle
  • Lack of engine power

What Are The Causes Of P0306?

  • Failed ignition coil
  • Bad spark plug
  • Defective/clogged fuel injector
  • Engine vacuum leak
  • Valvetrain problem
  • Piston/ring damage

How Serious Is The P0306 Code?

P0306 is a very serious code. It should be properly diagnosed and repaired right away. A misfiring engine can allow raw fuel to enter the exhaust system and cause premature catalytic converter failure or other engine damage.

How To Diagnose And Fix The P0306 Code

There are many possible causes for a P0306 code. As usual, it’s a good idea to start with the easiest, most accessible tests before diving into the more invasive engine mechanical tests.

First, you will want to scan the ECM using an OBD2 scan tool and record all codes and freeze frame data. Once you have this information a quick visual inspection of the ignition coils, fuel injectors, and related wiring. Look for any obviously damaged wires or loose connections. If nothing is found, a simple way to verify the condition of the ignition coil is to remove the coil from cylinder 6 and swap it with another cylinder. Then clear trouble codes and run the engine. Using your scan tool to check for DTC’s and see if the misfire follows the coil or stays on cylinder 6. If you have a higher-end scan tool you may be able to look at a misfire counter to see which cylinder is misfiring, instead of waiting for the ECM to flag a code. If the misfire followed the moved coil, replace the coil. If not, continue with your diagnosis.

Once the ignition coil is ruled out, a visual check of the spark plug should be performed. If there are any signs of damage or fouling it is a good idea to replace it, regardless if it’s the source of the problem or not. If you have any doubts, the spark plug can be swapped to another cylinder and see if the misfire follows using the same procedure as we did for the ignition coil.

Once the ignition system is ruled out, checking for vacuum leaks related to cylinder number 6 should be your next step. If nothing can be found visually, pull up the short term fuel trim PID on your scan tool for the engine bank containing cylinder 6. On a normal running engine, this number should stay around 0, anything over 0 means that the ECM is adding fuel to compensate for a lean condition. Any negative numbers indicate the ECM is taking away fuel to compensate for a rich condition. You can monitor these numbers and carefully spray carb cleaner around different sections of the engine, paying special attention to the intake manifold gasket area. If you see the fuel trim spike negative, that means you have found a vacuum leak. The ECM sees the carb clean as extra fuel and forces the trims negative to compensate. You can also use an unlit propane torch to be more precise. A smoke machine, if available is also a great way to check for vacuum leaks.

Next, the fuel injector should be inspected. Use a stethoscope to listen for clicking noise from the body of the number 6 injector while the engine is running. If you hear clicking it is safe to assume the circuit is operating. If not, check for a constant 12 volt signal on one wire and a pulsing ground on the other. If you are using a test light, it should pulse on the ground wire while the engine is running. If all of these tests pass, you can swap the injector with another cylinder and check for the following misfire with a scan tool the same as with the ignition coil and spark plug.

If no problems are found so far, it is likely there is a mechanical problem somewhere in the engine. Compression and/or leak-down test should be performed to verify. If compression is low, or the leak-down numbers are out of specification, parts of the engine will need to be disassembled. Some possible issued here could be a broken valve spring, burnt valve, carboned valve, worn piston rings, damaged piston, or a worn camshaft lobe.

Common Mistakes To Avoid While Diagnosing The P0306 Code

It is very common for people to replace the ignition coil and spark plug to try in repair a P0306. While ignition coils are the most common source of a misfire, it’s important to follow a proper diagnostic process to fix it right, without spending extra money on unnecessary parts.

Tips To Avoid P0306 In The Future

Following the manufacturer’s maintenance schedule will help avoid misfires. This keeps the engine in the best mechanical condition possible, and the spark plugs will be replaced at the proper intervals.

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