P0300 means random misfires across several cylinders, while P0301–P0304 point to a specific cylinder (the last digit is the cylinder number). The engine may shake, stumble under acceleration, and the check engine light may flash.
Why it matters
When a cylinder misfires, unburnt fuel goes into the exhaust. It overheats and can destroy the catalytic converter, and it can also dilute the engine oil. If the light is flashing, don't drive far or under heavy load.
Most common causes
- Spark plugs: worn or incorrectly gapped. Replacement intervals are usually 30,000–100,000 km depending on the engine.
- Ignition coils: pencil coils often fail, especially in damp weather.
- Fuel injectors: a clogged or leaking injector causes a single-cylinder misfire.
- Air leaks: a cracked vacuum hose or intake gasket leans out the mixture, often together with P0171.
- Mechanical: low compression (valves, rings, head gasket) – less common but more expensive.
- Diesels: injectors, or glow plugs on cold starts.
A simple test when one cylinder is flagged
If the code is, say, P0302, carefully swap the ignition coils of cylinders 2 and 3, clear the code and drive. If the fault moves to P0303, the coil is to blame. If it stays on P0302, check the spark plug next and then the injector. Only do this on a cold engine and label the connectors so they go back correctly.
What a random P0300 suggests
When misfires jump between cylinders, look for something they all share: low fuel pressure, a dirty or faulty mass air flow sensor, a large vacuum leak, or worn plugs across the board. Fuel trims in the live data help narrow this down quickly.
When to visit a workshop
If plugs and coils are fine, the next steps are a compression or leak-down test and injector checks. Misfire counters per cylinder in live data make the diagnosis much faster than swapping parts at random.
Enter your code and symptoms into Diagno's AI diagnosis to see the likely causes, and let a trusted mechanic do the compression and injector measurements if needed.