P0300 Fault Code: What It Means and What To Do

What does P0300 mean?

P0300 means your engine's control unit has detected misfires occurring randomly across more than one cylinder. A misfire happens when the air/fuel mixture in a cylinder fails to ignite properly — or ignites at the wrong time. Because the misfires aren't isolated to one cylinder (that would be P0301, P0302, etc.), the ECU logs P0300 to indicate the problem is either widespread or bouncing between cylinders.

You'll often notice P0300 through rough, lumpy idling, a juddering feeling under acceleration, reduced power, or a flashing engine warning light. On some cars the light will flash rapidly during active misfiring and then stay steady once the misfire becomes intermittent.

Common causes

• Worn spark plugs — the single most common cause. Plugs should be replaced every 30,000–60,000 miles depending on type. Worn plugs struggle to ignite the mixture reliably.
• Faulty ignition coil(s) — coils deliver the spark. A weak or failing coil causes intermittent misfires, often on multiple cylinders if the coil serves more than one plug.
• Bad fuel injectors — a clogged or leaking injector delivers too little or too much fuel to the cylinder, disrupting combustion.
• Vacuum leak — unmetered air entering the intake upsets the air/fuel ratio engine-wide, causing lean misfires across multiple cylinders.
• Low fuel pressure — a failing fuel pump or clogged fuel filter starves multiple cylinders at once.
• Carbon buildup on intake valves — common on direct-injection engines (VAG group, Ford Ecoboost, BMW). Carbon deposits disrupt airflow into the cylinder.

Typical UK repair costs

UK labour rates are typically £80–£120 per hour. Costs vary by vehicle and region.

What to do next

If the warning light is flashing: stop driving as soon as it is safe to do so. Active misfiring pumps unburnt fuel into the catalytic converter, which can destroy it in minutes — a catalytic converter replacement costs £300–£1,000+.

If the light is steady: book it into a garage soon. The first step is to check when the spark plugs were last replaced — if they're overdue, start there. An OBD reader can tell you which cylinder is misfiring most often, which helps narrow down the cause.