P0300: Random/Multiple Cylinder Misfire Detected

What does P0300 mean?

The ECM watches crankshaft speed. Every time a cylinder fires, the crankshaft speeds up slightly. If a cylinder fails to burn its mixture properly, that small boost is missing, and the ECM counts it as a misfire.

When misfires happen on several cylinders, or the ECM cannot pin them to one cylinder, it stores P0300. If one cylinder is to blame you will usually also see a code from P0301 to P0312, where the last digit is the cylinder number.

A flashing check engine light means the misfire rate is high enough to damage the catalytic converter. Unburned fuel can overheat it quickly.

Common symptoms

• Rough idle or shaking, especially at a stop
• Hesitation, jerking or loss of power when accelerating
• Flashing check engine light during heavy misfires
• Poor fuel economy
• Smell of fuel from the exhaust
• Hard starting or stalling in some cases

Common causes

The causes below are the ones commonly seen with P0300. The actual cause can vary by vehicle, engine and mileage, so treat this list as a starting point, not a diagnosis.

• Worn spark plugs or faulty ignition coils (often on more than one cylinder)
• Vacuum leaks or a lean condition affecting all cylinders
• Low fuel pressure or dirty injectors
• A faulty crankshaft or camshaft sensor signal, or timing problems
• Low compression from mechanical wear
• Contaminated or poor quality fuel
• Wiring problems affecting coils or injectors

What to check with a scanner

• Read all codes. P0300 together with P0171 or P0174 often points to a lean condition rather than ignition parts.
• Check Mode 06 misfire counters per cylinder if your vehicle supports them. They can show which cylinders are misfiring even when only P0300 is stored.
• Look at freeze frame data: rpm, load and engine temperature can tell you if misfires happen cold, at idle or under load.
• Watch fuel trims in Live data for signs of a lean or rich condition.

How to read and clear P0300 with OBD Master Pro

1. Plug in your adapter. Connect a generic ELM327 (or ELM329) OBD2 adapter to the diagnostic port, usually under the dashboard on the driver's side. Bluetooth Classic, Bluetooth LE, Wi-Fi and USB OTG adapters are supported.
2. Switch the ignition on. Turn the key to ON or start the engine, depending on your car.
3. Connect in OBD Master Pro. Open the app, go to Connection and pick your adapter. Classic Bluetooth adapters connect from the Bluetooth tab, Bluetooth LE adapters from the BLE tab with no pairing needed. Use the generic OBD2 / EOBD profile or your car's brand profile.
4. Read the codes. Open the fault codes screen to see stored, pending and permanent codes. Confirm P0300 is listed and note any other codes, since they often explain the cause.
5. Check the details. Look at freeze frame data, Live data and, where your car supports it, Mode 06 test results (see the checks above).
6. Fix, then clear. After the repair, clear the codes to turn off the check engine light. Clearing also resets the emissions readiness monitors, so drive normally and check the readiness screen before any inspection. Permanent codes cannot be erased with a scan tool; they clear on their own once the car's self test passes.

Tip: you can also look up P0300 and other codes in the app's DTC search without connecting to a car. If your adapter will not connect, use Connection > Share connection log and email it to support. Many cheap ELM327 clones only handle basic engine OBD2, which is all you need for P0300.

When to see a mechanic

Get professional help if any of these apply:

• The check engine light is flashing
• The engine shakes badly or loses power
• You smell fuel or see smoke
• The misfire continues after replacing plugs or coils
• You suspect a mechanical or timing problem

If you are unsure, a qualified technician can confirm the cause with tests such as smoke testing, pressure testing or scope readings. Clearing a code does not repair the fault.

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