P0300: Random or multiple cylinder misfire

What P0300 means

Verified description: Random/Multiple Cylinder Misfire Detected

General OBD-II powertrain explanation for vehicles that support P0300. The examples and checks here focus on gasoline engines; diesel diagnosis differs. This library entry does not establish UTH Scanner compatibility with your vehicle or its modules.

Vehicle-specific limits

• Detection thresholds, cylinder identification, test conditions and repair procedures depend on the exact year, model and engine. Use the vehicle-specific service information before testing components.
• A service bulletin is not a universal repair. For example, Ford TSB 13-1-11 concerns a narrowly defined group of 2013 Mustang 5.0L manual-transmission vehicles with particular stored-code conditions; it does not justify a software update or parts replacement on another vehicle.

What the code does and does not tell you

What it establishes

• The engine controller recorded a misfire-monitor fault under its programmed conditions.
• The stored code and any freeze-frame data are useful evidence to preserve for diagnosis.

What it does not establish

• That spark plugs, ignition coils, fuel injectors or the engine computer have failed.
• Which cylinder needs repair, whether the fault is still active, or whether continuing to drive is appropriate.
• That clearing the code fixes the cause.

Symptoms you may notice

• A check-engine light, which may flash while a damaging misfire is occurring.
• Uneven idle, shaking or rough running; some vehicles have no obvious running complaint when the code is found.

Possible causes

• An ignition problem, fuel delivery or injector problem, or a mechanical compression problem can interrupt combustion.
• Certain engine-control or emissions faults can also contribute. The relevant causes must be narrowed using the other codes, symptoms and vehicle-specific tests.

A possibility is not a diagnosis. Confirm the cause with the vehicle-specific testing procedure before replacing parts.

Sensible next checks

1. Record every code, its status, and available freeze-frame data before clearing anything. Note whether the problem happens at cold start, idle or under load, and whether it began after refueling or recent service.
2. Check the owner's manual for the warning-light instructions. With the engine off and cool, look only for obvious loose connectors or intake hoses after recent work; keep clear of hot, moving and high-voltage parts.
3. Ask a technician to compare misfire information with fuel-system data and related codes, then perform the appropriate ignition, fuel or mechanical tests. Do not buy a set of coils solely because P0300 is present.

When to get professional help

Arrange diagnosis promptly if P0300 returns, the engine runs poorly, or additional codes are present. Fuel-system, ignition and compression testing call for the correct tools and vehicle procedure. Preserve the evidence instead of repeatedly clearing the warning.

Related codes

• P0301 — Cylinder 1 Misfire Detected
• P0302 — Cylinder 2 Misfire Detected
• P0171 — System Too Lean (Bank 1)
• P0174 — System Too Lean (Bank 2)