P0305

What P0305 means

The computer tracks how smoothly the engine spins by watching the crankshaft speed up between cylinder firings. When cylinder 5 isn't contributing its share of power, that cylinder's stroke shows up as a small stumble the computer can detect. P0305 means it has counted enough of those stumbles from cylinder 5 specifically to call it a misfire. It doesn't tell you why the cylinder is misfiring, only that it is.

Is it safe to drive

Stop driving it, or at least get it looked at right away. A misfire means unburned fuel is passing 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. If the check engine light is flashing rather than steady, that's the car telling you this is happening now and actively causing damage. Don't keep driving on a flashing light. A steady light is less urgent but still worth getting in soon. Watch for a rough idle, a hesitation or jerk under acceleration, or a smell of raw fuel from the exhaust.

Common causes

Most often it's something in the ignition path for that one cylinder: a worn spark plug, a failing ignition coil (on engines with one coil per cylinder), or a cracked spark plug wire on older coil-and-distributor setups. Next most common is a fuel delivery problem specific to that cylinder, usually a clogged or weak fuel injector. Beyond that, a vacuum leak affecting that cylinder's intake runner, or low compression from a worn valve or head gasket issue, can cause the same code. On some engines a bad coil on a neighboring cylinder can cross over and cause a misfire code on cylinder 5 too, which is one reason we don't just replace parts based on the code alone. The right cause also depends on the engine. Some have one coil per plug, some fire two cylinders off one coil, and that changes where we look first.

Diagnosis and rough cost

We start by pulling the code data to see how often the misfire happens and under what conditions, then check spark, then check fuel delivery to that cylinder, then check compression if the first two look fine. On a lot of engines, swapping the coil or plug from cylinder 5 to another cylinder and seeing if the misfire follows it is a quick way to confirm the coil is the problem rather than guess. A single bad coil or plug is a fairly inexpensive fix, often in the range of a couple hundred dollars including parts and labor. A bad injector costs more. If it turns out to be a compression problem inside the engine, that's a bigger job and the cost goes up a lot from there. We won't know which one it is until we actually test it, so we're not going to quote a number before we look.

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.