Draft guidance: awaiting Claude spot-check, qualified-mechanic review and named-editor approval. Do not use a fault code alone to replace parts.

P0300: Random or multiple-cylinder misfire detected

The engine control unit has detected misfiring that affects more than one cylinder, occurs irregularly, or cannot yet be assigned reliably to one cylinder.

What P0300 means

The engine control unit has detected misfiring that affects more than one cylinder, occurs irregularly, or cannot yet be assigned reliably to one cylinder. P0300 identifies an uneven-combustion problem, not the component that caused it.

Can I still drive?

Treat an active misfire as urgent. A flashing engine-management light can mean the misfire is severe enough to overheat and damage the catalytic converter. Stop as soon as it is safe, switch off and arrange recovery if the light flashes or turns red, the engine shakes strongly, power falls substantially, the vehicle stalls, or there is smoke, overheating, an abnormal mechanical noise or a strong smell of unburnt fuel. If the light is steady amber, the engine feels normal, fluid levels are satisfactory and there are no other warning signs, a short and gentle journey to a nearby garage may be reasonable. Avoid motorways, high engine speed, hard acceleration and heavy loads.

Symptoms

Steady or flashing engine-management light
Rough or uneven idle
Juddering, hesitation or surging under acceleration
Reduced power
Difficult starting or intermittent stalling
Increased fuel consumption
Unusual exhaust smell or smoke
No obvious symptom if the fault is intermittent

Common causes

Worn, fouled, incorrectly specified or damaged spark plugs on petrol engines
Weak ignition coils, a failing shared coil pack, damaged HT leads or ignition wiring
Restricted, leaking or electrically faulty fuel injectors
Low or unstable fuel pressure, contaminated fuel or a fuel-supply fault
Unmetered air from a split intake or vacuum hose, manifold leak or air-metering problem
Incorrect air-fuel mixture caused by a sensor or control fault
Low compression caused by valves, piston rings, a cylinder-head gasket or valvetrain fault
Incorrect camshaft or ignition timing
Wiring, connector, earthing or control-unit software faults

Safe first checks

Record every stored and pending code plus freeze-frame data before clearing anything. Fuel-trim, injector, timing or sensor codes may change the diagnostic direction.
With the engine switched off and cool, check the oil and coolant levels. Never open a hot cooling system.
Look for an obviously disconnected air hose, split intake pipe, loose electrical connector or visible fluid leak.
If fuel is leaking or strongly smelt, do not start or drive the vehicle.
Check the service history for overdue spark plugs, filters or timing maintenance, and note whether the fault appears on cold start, in rain, at idle or under load.
Leave ignition-output tests, fuel-pressure work, injector testing, compression tests and timing checks to a competent technician.

Typical UK fix cost

Allow roughly £100–£600 for diagnosis and a common repair such as a plug set, an accessible coil or coil pack, a minor intake leak or an injector-related repair. Timing, cylinder-head, internal-engine or catalytic-converter damage can raise the total to £600–£2,500+. This is a deliberately broad budgeting range, not a quotation. A human reviewer should check the exact engine, diagnosis, parts quality, labour, VAT and whether the quote covers one component or a full set.

Related models

This is a generic OBD-II code used across many makes and models. The exact test thresholds, component layout and repair route vary by engine and manufacturer, so check vehicle-specific technical information before ordering parts.

Related fault codes

Sources

A diagnostic trouble code identifies a system or circuit to investigate. It does not prove that a named part has failed. Preserve freeze-frame data and seek a vehicle-specific diagnosis before clearing the code.