P0306

What P0306 means

Your engine has six or more cylinders, and the computer has detected that cylinder number 6 is misfiring. That means the air-fuel mixture in that cylinder isn't burning the way it should, or isn't burning at all on some cycles. The computer catches this by watching small fluctuations in crankshaft speed. Each cylinder is supposed to contribute an even push. When one doesn't, the code sets.

Is it safe to drive

Drive it as little as possible until this gets looked at. A misfire sends unburned fuel into the exhaust, and that fuel can overheat and destroy the catalytic converter, which is a much more expensive repair than whatever caused the misfire in the first place. If the check engine light is flashing rather than steady, that's the car telling you the misfire is active and bad enough to cause damage right now. Stop driving it and get it towed or diagnosed as soon as you can. If the light is steady, you can usually drive it to us, but don't put off the appointment.

Common causes, most likely first

On most engines, the first things to check are the spark plug and ignition coil for cylinder 6, since those wear out and fail more often than anything else in the ignition system. After that, a fuel injector for that cylinder can be clogged or failing. Vacuum leaks near that cylinder, low compression from a valve or piston ring problem, or a bad head gasket can also cause a misfire, though those are less common. On engines with individual coils per cylinder, a coil failure tends to show up as a single-cylinder code like this one. On engines with a distributor or shared coil packs, a misfire on just one cylinder more often points to that cylinder's plug, injector, or a mechanical issue rather than the ignition system as a whole. Which of these it is depends on the engine, and we won't know for sure until we test it.

What diagnosis involves

We start by pulling the plug from cylinder 6 to look at its condition, and often we'll swap the coil with another cylinder to see if the misfire follows the part or stays put. That alone tells us a lot. If the plug and coil check out, we'll look at fuel delivery to that cylinder and run a compression test to rule out a mechanical problem. A basic diagnosis usually runs somewhere in the range of an hour of shop time, sometimes less if the cause is obvious once we start looking. The repair itself varies a lot: a plug is inexpensive, a coil is more, and if it turns out to be a compression or head gasket issue, that's a bigger job. We won't recommend replacing the coil or injector until we've actually confirmed that's the cause, not just guessed based on the code.

Generic OBD-II guidance — manufacturer-specific codes and causes vary by make, so always confirm against your vehicle. A code identifies a system or condition; it does not, on its own, prove which part has failed.