P0300 — random / multiple cylinder misfire

What P0300 actually says

P0300 is the generic OBD-II code for a random or multiple cylinder misfire. It is stored when the engine computer sees the crankshaft slow momentarily in the pattern a failed combustion event produces, and cannot attribute those events to one cylinder.

The attribution rule is written into the regulation, which is why the presence or absence of a cylinder code carries real information: a cylinder-specific code is stored when more than 90 % of misfires come from one cylinder, and a multiple-cylinder code is stored when two or more cylinders each account for more than 10 % of them.

Flashing versus steady

The regulation requires the light to blink or flash once per second while catalyst-damaging misfire is occurring, and that fault sets on a single trip rather than waiting for a second one.

So a flashing light is not a more urgent version of the same message — it is a different message. Keep driving and unburned fuel keeps reaching the converter.

You may read elsewhere that “if P0300 is your only code the light will not be flashing.” That is wrong. A catalyst-damaging misfire spread across cylinders stores P0300 and flashes the light.

Which cylinder is it?

P0300 names none, by design. The honest answer is a procedure, not a part: read live misfire counters, then confirm by swapping a coil to another cylinder and seeing whether the misfire follows it.

Cylinder numbering is not universal across manufacturers, and waste-spark systems fire in companion pairs, so a single failing coil can misfire two cylinders that are nowhere near each other. This tool never tells you where a numbered cylinder physically sits.

If you also have P0420

Fix the misfire first. Raw fuel from a misfire is what destroys a converter, so a converter replaced while the misfire is still there is on the same path as the one it replaced.

How the ranking works

Each answer fires named rules that move candidates up or down. Nothing is random and nothing is weighted by what a part costs. The same answers always produce the same order, and every row shows the rules that put it there.

Confidence has a definition rather than a feeling:

• High — something you told us points specifically here, and nothing argues against it.
• Moderate — your answers support it, but nothing separates it decisively.
• Low — nothing in your answers points here; it is listed because it is a common cause.

What this tool does not know

• It has not scanned your vehicle. Everything it has is what you typed.
• It does not know your engine's cylinder layout, wiring, or which coil serves which cylinder.
• It cannot see fuel pressure, compression, or live data unless you enter them.
• Costs are ranges, and real prices vary with vehicle, region, and labour rate.
• It narrows where to look. It does not replace hands-on testing, and a flashing light means stop driving regardless of what it says.