Code

P0306 – Misfire detected on cylinder number 6.

Stop Driving

• If the check engine light is blinking, stop driving immediately.
• If the check engine light is steady, you have time to find a comfortable place to stop and diagnose the issue.
• No matter what, do not drive for long with a misfire. It can damage expensive exhaust and intake components.

Related Sensors

• ECM Misfire Detector
• Crankshaft Position Sensor

Accompanying Symptoms

A misfire will often cause the following symptoms.

• Poor acceleration
• Jerky movement while accelerating
• Unusual odors
• Unusual noises
• Rough idle

What Is a Misfire?

A misfire occurs when the combustion is weak, incomplete, or does not occur. Your car’s engine is constantly sucking in air and fuel, compressing them, then igniting them to create combustion. During a misfire, some part of the equation is lacking. Either the air to fuel ratio is incorrect, compression is not occurring sufficiently, or the ignition spark is inadequate.

Air/Fuel

Your car’s engine uses various sensors to ensure the air to fuel mixture is about 14.7:1. That is the optimal ratio for a standard gas motor. If the mixture is too rich, it means that there is too much fuel. In contrast, a lean mixture has too little fuel.

A lean fuel mixture can cause a misfire. If there is not enough fuel in the mixture, it can not ignite. This can be caused by various factors, including a bad MAF sensor, clogged fuel injectors or other fuel delivery issue, or a dirty air filter providing an inaccurate airflow reading.

Exhaust

It is possible, though unlikely, for exhaust gases to cause mixture problems. If the exhaust gases cannot escape, they will be compressed with the air and fuel. Because exhaust gasses can not burn, combustion will not occur; a stuck valve or blocked exhaust pipe can cause this scenario.

Compression

The second cycle of a 4 stroke engine is the compression cycle. The piston squeezes the air and fuel to maximize potential energy. If the cylinder is not airtight, then the air and fuel will not compress. The gasses will “blow by” the piston rather than get squeezed.

Compression failure is one of the more serious problems that can cause a misfire. Either the piston is not sealing against the cylinder walls, or there is a valve issue. A leak down test or compression test can diagnose compression issues, but physically inspecting the internal components is the only way to know the cause.

Ignition

Ignition issues used to be the most common causes of misfires. Older cars used a distributor that had to be timed to the engine manually. If it was timed incorrectly, the spark would occur at the wrong time.

Modern cars do not have distributors, though. Instead, they have individual coils that are controlled by the computer. If there is a spark issue causing a misfire in a modern car, it is usually because either the spark plug or coil pack is not functioning correctly.

It is less common, but ignition problems can also occur when other sensors have failed. If a crank position sensor has failed, for instance, then the computer may believe the piston to be in the ideal position for ignition without it being true.

Common Issues

The following represents a good place to start when you begin to diagnose a misfire. Note that since this code specifically concerns cylinder number 6, you can safely eliminate it whenever an issue could affect all cylinders.

• Check the spark plug. A fouled or broken spark plug is a very common issue that causes misfires.
• While you are checking the spark plug, inspect the boot and ignition coil for damage.
• Check your air filter. A bad air filter can cause issues with the sensors and create non-optimal air:fuel mixtures.
• Check your fuel filter. A dirty fuel filter can cause inadequate fuel pressure and lean out the mixture.

Advanced Diagnostics

Some issues that can cause this error code require more advanced diagnostic methods. These can be done by any mechanic shop, but average garages usually lack the tools to perform all the tests.

• Use a smoke machine to check for vacuum leaks. A vacuum issue can cause all sorts of issues, including incorrect timing and incorrect sensor readings.
• Use a leak down or compression test to make sure that cylinder 6 has adequate compression.
• Use a fuel pressure gauge to make sure the fuel rails have an adequate supply of gas.
• As a final step, begin removing exhaust components and inspecting them for obstructions.

Solutions

Generally, the solution will be to replace the faulty part. Little can be done to repair a spark plug or ignition coil. Most of these parts are cheap, and it’s a good idea to regularly inspect and replace many of them anyway. Filters are especially important to regularly replace.

The exception is if the motor fails a compression test. Failing a compression test means you will have to perform extensive surgery on the motor to either remove the piston or re-hone the valve seats. Removing the heads from a motor is not for the faint of heart. For many cars, it may be easier to buy a new motor rather than repair a motor that has low compression.