P0300 is a random or multiple-cylinder misfire. P0301 through P0312 name a specific cylinder — the last two digits are the cylinder number, so P0304 is cylinder 4.
The distinction is the whole diagnosis. A named cylinder points at that cylinder's own parts. An unnamed one points at something every cylinder shares.
How the module detects a misfire at all
It does not look inside the cylinder. It watches the crankshaft position sensor and measures how much the crankshaft accelerates on each power stroke.
A cylinder that fires normally gives the crank a shove. One that misfires does not, and the crank momentarily slows. The module knows the firing order, so it knows which cylinder should have produced the shove that did not arrive.
Two consequences follow. First, the detection is precise enough to name a cylinder. Second, a rough road can produce enough crankshaft speed variation to confuse it, which is why some vehicles suspend misfire detection when the ABS wheel sensors report a rough surface.
When a cylinder is named
P0301 to P0312 point at the three things that cylinder alone controls:
- Spark — the coil, the plug, the boot, the plug wire on older engines
- Fuel — that cylinder's injector, electrically or mechanically
- Compression — valves, rings, head gasket, and timing if it has slipped
The classic first test costs nothing. Swap the coil with a neighbouring cylinder, clear the code, and drive it. If the misfire moves with the coil, the coil is the fault. If it stays on the original cylinder, the coil is fine and the cause is fuel or compression. Do the same with the injector next.
That test answers in one step what parts-swapping answers in three purchases.
When no cylinder is named
P0300 means several cylinders are misfiring, or the module could not attribute the misfires. Look at what they share:
- Fuel delivery — pressure, the pump, a blocked filter
- A large vacuum leak — an intake gasket, a split hose, a failed brake booster
- Ignition timing — a stretched timing chain, a slipped belt, a crank or cam sensor
- Fuel quality — contaminated or old fuel, or water in the tank
- EGR stuck open, diluting the charge at idle
P0300 arriving together with several named cylinder codes usually means one shared cause severe enough that individual cylinders crossed the threshold too. Work the shared cause.
Read the fuel trims
Long-term fuel trim well positive across the board points at a lean condition — an air leak or fuel delivery — which fits P0300 far better than replacing every coil.
Trims near zero with a strong misfire point away from mixture and towards ignition, timing or compression.
Why this family matters more than the rest
Misfire codes are the ones that make the check engine light flash, and a flashing light on a misfire means stop driving.
Fuel that does not burn in the cylinder burns in the catalytic converter instead. The converter is designed to run around 800°F; raw fuel pushes it far past that and the ceramic substrate melts. This takes minutes, and turns a coil-and-plugs job into a converter replacement.
If the light is flashing, reduce load, pull over safely, and have the vehicle recovered rather than driven. If it is steady, the misfire is intermittent or mild, and driving carefully to a workshop is reasonable.
Cold-start-only and load-only misfires
The freeze frame is worth reading closely here, because misfires that appear under one condition have short suspect lists.
A misfire only at cold start that clears as the engine warms points at fuel delivery when the engine is cold, or at a valve-seat or ring issue that seals better once the metal expands.
A misfire only under load points at ignition breaking down under cylinder pressure — a plug gap grown too wide, a coil weak enough to fire at idle but not against compression, or a plug boot tracking to earth.
A misfire only at idle that disappears above 2,000 rpm points at a vacuum leak, which matters proportionally more when total airflow is small.
Before buying anything
Check the plugs. They are usually the cheapest item in the chain, they wear on a schedule, and their condition tells you what has been happening: oil fouling, coolant deposits, a melted electrode, or simply a gap that has grown well past specification after 60,000 miles.
A set of plugs is often both the fix and the diagnostic reading, and it is the one part on the list you would eventually replace anyway.