P0300 and P0301 together: random misfire plus cylinder 1

The two codes

• P0300: Random or multiple-cylinder misfire detected
• P0301: Misfire detected in cylinder 1

Why they show up together

The computer stores a cylinder code (P0301 for cylinder 1) when it can pin the misfire to that cylinder, and P0300 when misfire is spread across more than one cylinder or it cannot tell which. Seeing both usually means cylinder 1 misfires most, with occasional misfire elsewhere.

That can happen two ways: a fault on cylinder 1 that is bad enough to unsettle the engine and trigger occasional misfire counts on other cylinders, or a shared problem (lean mixture, vacuum leak, fuel quality) that hits cylinder 1 hardest.

Likely primary: P0301

P0301 is the more specific code: it names a cylinder, so the parts that belong only to that cylinder (coil, plug, boot, injector) can be tested directly and quickly. If cylinder 1 is fixed and P0300 does not return, the random code was a side effect.

Shared causes

Diagnose in this order

1. Check the light first: if it is flashing, stop driving (see severity). Note any other codes, especially lean codes (P0171, P0174), because a lean mixture can cause misfire and should be fixed first.
2. Read misfire counters for each cylinder with a scan tool to see how the misfire is spread.
3. On coil-on-plug engines, swap the cylinder 1 coil with a neighbouring cylinder's coil and see whether the misfire moves. Repeat with the spark plug. If it follows the part, that part is the cause. Bolt swapped coils down so they keep their ground, and do this only with the engine off and cool.
4. If the misfire stays on cylinder 1, the next suspects are that cylinder's injector and its compression, which are tests for a shop.
5. If cylinder 1 checks out or several cylinders show counts, move to shared causes: vacuum leaks, fuel pressure and fuel quality.